Emperor Cheng of Jin (Chinese: 晉成帝; pinyin: Jìn Chéng Dì; Wade–Giles: Chin Ch'eng-ti; December 321 or January 322 – 26 July 342), personal name Sima Yan (司馬衍), courtesy name Shigen (世根), was an emperor of the Eastern Jin Dynasty (266–420). He was the eldest son of Emperor Ming and became the crown prince on April 1, 325. During his reign, the administration was largely dominated by a succession of regents—initially his uncle Yu Liang, then Wang Dao, then the joint administration of He Chong and another uncle Yu Bing (庾冰). He became emperor at age four, and soon after his accession to the throne, the disastrous rebellion of Su Jun weakened Jin forces for decades.