Ilkhanids is a state of the Mongol that was established by Hulagu Khan, the grandson of Genghis Khan, in Iran. Until the reign of Ghazan Khan, the Mongol traditions were influential rather than religions. The process of Islamization that began with Ahmad Tekudar, did not find enough reaction for the Ilkhanid society. The conditions of the reign and Tekudar's weak personality had been effective in this situation. The influence of Islam began in the reign of Ghazan. His close relationship with all the elements that make up society, prevented the conflict between Islamic sects. Oljeitu lived as a Buddhist until the age of seven. After that He was baptised. Later he converted to Islam. After the discussions between the Islamic sects, he chose Shiism as a sect. Oljeitu's sect situation had led to changes provided by Shi'ism in the structure of khutba (sermon) and azan. This had resulted in Shi'ism's being as the official sect of the state for the first time. There exist multiple elements about Oljaitu's Shi'sm. So, the Oljaitu's Shi'sm should not be explained only by religious reasons. This article aims to review reasons and results of Oljaitu's transition to Shi'ism.