320

320 (CCCXX) was a leap year starting on Friday of the Julian calendar, the 320th year of the Common Era (CE) and Anno Domini (AD) designations, the 320th year of the 1st millennium, the 20th year of the 4th century, and the 1st year of the 320s decade. As of the start of 320, the Gregorian calendar was 1 day ahead of the Julian calendar, which was the dominant calendar of the time.

• Crispus wins against the Franks.
• Licinius stopped the religious freedom promised by the Edict of Milan. He persecutes Christians in the Eastern Roman Empire.

• King Chandragupta I founds the Gupta dynasty in northern India.
• The Huns appear in Persia.

• Old St. Peter's Basilica, Rome, is started to be built (approximate date).

• 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, sees an eclipse of the sun. He writes on The Great Astronomer (Almagest).

• 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)

• Lactantius, Christian writer (approximate date)
• Sima Bao, prince of the Jin Dynasty (b. 294)