The 320s decade ran from January 1, 320, to December 31, 329.
- Crispus, eldest son of Constantine I, leads a victorious campaign against the Franks, assuring twenty years of peace along the Rhine frontier. He establishes his residence in Augusta Treverorum (modern Trier), capital of Germania.
- Licinius reneges on the religious freedom promised by the Edict of Milan, and begins a new persecution of Christians in the Eastern Roman Empire. He imprisons Christians, confiscates their properties and destroys churches.
- King Chandragupta I founds the Gupta dynasty in northern India.
- Zhang Shi (張寔), Zhang Duke of Xiping and governor of Liang Province, (涼州)is assassinated by Yan She (閻涉) and Zhao Ang (趙卬) and replaced by Zhang Mao (張茂), commonly accepted first ruler of the Chinese state Former Liang.
Culture and Religion
- In Nydam Mose (Denmark), the Nydam oak boat is sacrificed by the Danes (the boat is excavated in the 1830s, when a local farmer finds weapons, including swords and spears).
- December 25 is introduced as the birthday of Jesus.
- October 18 – Pappus of Alexandria, Greek philosopher, observes an eclipse of the sun and writes a commentary on The Great Astronomer (Almagest).
- Emperor Constantine I expels the Goths from the Danube frontier and repairs Trajan's Bridge. He leads an expedition into the old province Dacia (modern Romania) and makes peace with the barbarians.
- March 7 - Constantine I signs legislation directing urban residents to refrain from work, and businesses to be closed, on the "venerable day of the Sun". An exception is made for agriculture.
Arts and sciences
Food and drink
- Constantine I assigns convicts to grind Rome's flour, in a move to hold back the rising price of food in an empire whose population has shrunk as a result of plague (see 309 AD).
- The first dependable representation of a horse rider with paired stirrups is found in China, in a Jin Dynasty tomb.
- Emperor Constantine the Great defeats the invading Goths and Sarmatians north of the Danube in Dacia, and claims the title of Sarmaticus Maximus.
- July 3 – Battle of Adrianople: Emperor Constantine the Great defeats his rival Licinius near Adrianople, forcing him to retreat to Byzantium. He invades Thrace with a Visgothic force and raids the countryside. This sees Constantine ruling as sole Emperor.
- July – Battle of the Hellespont: Crispus destroys Licinius' naval fleet in the Dardanelles, allowing his father Constantine the ability to cross over the Bosphorus into Asian provinces. Byzantium is besieged and Licinius assembles a second military force, under his newly elevated co-emperor Martinian at Lampsacus (modern Lapseki).
- September 18 – Battle of Chrysopolis: Constantine I definitively defeats Licinius at Chrysopolis, and becomes sole Emperor, thus ending the period of the Tetrarchy. Licinius escapes and gathers around 30,000 of his surviving troops at Nicomedia.
- December 19 – Licinius abdicates his position as Emperor. He is pardoned by Constantine I as a result of the supplication of his wife Constantia (who is Constantine's halfsister), and banished to Thessalonica as a private citizen.
- Emperor Constantine I personally assures the security of the Danube frontier by defeating the Goths, the Vandals, and the Sarmatians.
- Licinius is executed in Thessalonica, on charge of conspiring and raising troops against Constantine I.
- Gladiatorial combat is outlawed in the Roman Empire.
- The First Council of Nicaea is held.
- April 1 – Crown Prince Jin Chengdi, age 4, succeeds his father Jin Mingdi as emperor of the Eastern Jin dynasty. During his reign he is largely advised by regents, his uncle Yu Liang and high-level officials.
- Constantine the Great, from the Basilica of Maxentius and Constantine, Rome, is started to be made. It is now kept at Palazzo dei Conservatori, Rome.
- May 20 – First Council of Nicaea: Constantine I summons an ecumenical council of bishops in Nicaea (Turkey). The Nicene Creed, adopted on June 19, declares that the members of the Trinity (the Father, the Son, and the Holy Spirit) are equal. The council decides that Easter is celebrated on the first Sunday after the first full moon after the vernal equinox. Arius is exiled to Illyria; his works are confiscated and consigned to the flames.
- The Church of the Nativity is built in Bethlehem.
- Emperor Constantine the Great travels to Rome to celebrate the 20th anniversary of his accession to power, but while en route at Pola he orders his older son, Crispus Caesar, to be executed, possibly on charges of adultery. Fausta, second wife of Constantine I, is also executed by being suffocated in a hot bath.
- Constantine I founds Constantinople and incorporates Byzantium into the new capital. He reorganises the Roman army in smaller units classified into three grades: palatini, (imperial escort armies); comitatenses, (forces based in frontier provinces) and limitanei (auxilia border troops).
- Constantine I promulgates laws against the prostitution of maidservants, and for the humanization of prisons.
- Constantine the Great, from the Basilica of Maxentius and Constantine, Rome, is finished. It is now kept at Palazzo dei Conservatori, Rome.
- September 14 (traditional date) – Helena, mother of Constantine I, discovers the so-called True Cross and the Holy Sepulchre (Jesus's tomb) in Jerusalem. On her pilgrimage, she pauses on the Aegean island of Patmos, where she is said to found the church of Panagia Ekatontapiliani.
- Helena tells Constantine that he must atone for executing his son and wife by building churches, and at about this date construction begins on Old St. Peter's Basilica, the first church on the traditional site of Saint Peter's tomb in Rome, and on the basilica of Golgotha on Calvary outside Jerusalem.
- Probable date – Christianity is introduced to the Kingdom of Iberia (modern-day Georgia) by Saint Nino.
- Emperor Constantine the Great decrees that rural slaves can only be sold in the province where they reside, in order to resolve the shortage of labour in the Roman Empire.
- Construction begins on the cathedral of Antioch (Syria).
- Approximate traditional date – Helena, mother of Constantine, returning from her pilgrimage to the Holy Land, founds Stavrovouni Monastery on Cyprus.
- July 5 – Constantine's Bridge, built over the Danube between Sucidava (Corabia, Romania) and Oescus (Gigen, Bulgaria), is officially opened by the Roman architect Theophilus Patricius.
- December 7 – Lakhmid king Imru' al-Qays ibn 'Amr dies. His epitaph, the Namara inscription, is an important source for the Arabic Language.
- Roman restrictions on joining the clergy are initiated.
- Aurelius Victor, historian and politician (approximate date)
- Constans I, Roman emperor (d. 350)
- Flavian I, Patriarch of Antioch (d. 404)
- Jin Jianwendi, emperor of the Jin Dynasty (d. 372)
- Oribasius, Greek physician (approximate date)
- Tuoba Shiyijian, prince of the Tuoba Dai (d. 376)
- Xie An, statesman of the Jin Dynasty (d. 385)
- July 3 – Valentinian I, Roman emperor (d. 375)
- Du Lingyang, empress of the Jin dynasty (d. 341)
- Jin Chengdi, emperor of the Jin dynasty (d. 342)
- Ammianus Marcellinus, Roman historian (approximate date)
- Procopius, usurper of the Roman Empire (approximate date)
- Wang Meng, prime minister of the Former Qin (d. 375)
- Constantius Gallus, Caesar and son of Julius Constantius (d. 354)
- Murong Chui, general and founder of Later Yan (d. 396)
- Flavius Julius Valens, Roman Emperor (d. 378)
- Huan Chong, general and governor of the Jin Dynasty (d. 384)
- Yuan Hong, historian and politician of the Jin Dynasty (d. 376)
- January 2 – Narcissus, Argeus, and Marcellinus, Roman Catholic martyrs and saints
- January 29 – Valerius of Trèves, Roman Catholic priest and saint
- February 6 – Dorothea of Alexandria, Roman Catholic virgin, martyr and saint
- March 9 – 40 Martyrs of Sebaste
- March 10 – Cyrion and Candidus, Armenian Orthodox priest and saints
- July 11 – Januarius and Pelagia, Roman Catholic priests, martyrs and saints
- November 29 – Saint Illuminata, Roman Catholic religious sister and saint
- December 9 – Proculus of Verona, Roman Catholic priest and saint
- Lactantius, Christian writer (approximate date)
- Sima Bao, prince of the Jin Dynasty (b. 294)
- Zhang Shi, Duke of Xiping
- Abibus of Edessa, Christian Deacon
- Raba bar Rav Huna, Jewish Talmudist
- Yang Xianrong, Chinese empress
- January 3 – Jin Yuandi, emperor of the Jin Dynasty (b. 276)
- Zhang Bin, Chinese strategist of the Jie state Later Zhao
- Guo Pu, Chinese writer (b. 276)
- Wang Dun, general of the Jin dynasty (b. 266)
- Zhang Mao, ruler of Former Liang (b. 277)
- October 18 – Jin Mingdi, emperor of the Jin dynasty (b. 299)
- Iamblichus, Syrian philosopher (b. 245)
- Licinius, Roman emperor (executed)
- Sextus Martinianus, Roman Emperor (executed)
- Tuoba Heru, prince of the Tuoba Dai
- Alexander, Patriarch of Alexandria
- Crispus, son of Constantine I (executed)
- Fausta, second wife of Constantine I (executed)
- Empress Liu
- June 3 – Awtel, Eastern Christian monk and saint
- October 19 – Saint Cleopatra, Christian saint of the Coptic Orthodox Church
- Betause, Bishop of Reims
- Alexander of Alexandria, bishop of Alexandria
- Su Jun, general of the Jin Dynasty
- Yu Wenjun, empress of the Jin Dynasty (b. 297)
- Liu Xi, emperor of Han Zhao
- Liu Yao, emperor of Han Zhao
- Liu Yin, prince of Han Zhao
- Wen Jiao, general and governor of the Jin Dynasty