Sima Zhao died in September 265, and was succeeded as regent of Cao Wei by his son
Sima Yan. In February 266, Sima Yan accepted
Cao Huan's abdication and founded the Jin dynasty. Sima Gan was then made Prince of Pingyuan, with a fiefdom of 11,300 households. Although his fiefdom was large, he did not take an active role in administration; instead, he appointed officials based on their abilities and the positions which needed to be filled. He also allowed his income (cloth, silk) to accumulate and rot. Sima Yan died in May 290, and his son
Sima Zhong ascended the throne. During audiences with the emperor, as the emperor's great-uncle, Sima Gan was allowed to wear his shoes and carry his sword, as well as walk at a normal pace. The
Book of Jin recorded several anecdotes between Sima Gan and his grandnephew
Sima Jiong. During the
War of the Eight Princes, after Sima Jiong defeated
Sima Lun, while other clansmen of the Sima family and court officials sent oxen and wine as gifts to Sima Jiong, Sima Gan offered him 100 cash. At their meeting, Sima Gan said, "The Prince of Zhao (Sima Lun) caused a rebellion and you managed to put it down; that is your credit. Now, I'm congratulating you using these 100 cash. Even so, it is very difficult to remain at a high position; you must be careful and cautious." After becoming regent, Sima Jiong once received a visit from his great-uncle. While Sima Jiong stepped out of the house and received Sima Gan with courtesy, upon entering the house, Sima Gan sat down on Jiong's bed and did not allow Jiong to be seated. Gan then said, "You must not emulate the son of that Bai woman (referring to Sima Lun)." After Sima Jiong was killed in January 303, Sima Gan cried bitterly and told those around him, "The Sima clan is waning; only this child is the most capable. Now that he has died, the Sima clan is in peril!" After
Sima Yue became Emperor Hui's last regent in 306, he decided to pay Sima Gan a visit at Gan's residence in Luoyang. Gan declined to meet him; after a lengthy wait, Gan despatched a servant to see Yue off. At the same time, Gan left the door ajar, allowing him to observe the scene from behind the door. At the time, people could not understand his actions; they thought that Gan was ill, or that he had turned reclusive. ==Descendants==