Upon the downfall of the Abbasid Dynasty and the establishment of the Ilkhanid government, the Twelver Shi’ites, who were not allowed to perform any religious activities during the Abbasid period, began disseminating Shi’ism with the help of some scholars such as Khwajah Nasir al-Din Tusi while benefiting from the compromising attitude of Mongols and their lack of religious prejudice. In the middle of the Ilkhanid period, a number of Ilkhanid kings converted to Islam and adopted the Sunnite tradition. However, because of the conflicts among Sunnite scholars, the enmity of Ilkhanid government with the Sunnite Mamluk Sultanate of Egypt, and the endeavors of Shi’ite statesman, Shi’ism assumed an important role in the country. After Őljeitű’s conversion to Shi’ism, the revival of Mongolian traditions in Iran was prohibited. The formal recognition of Shi’ism faced some resistance on the part of Sunnite statesmen and scholars of the time, and they decided to stop Őljeitű from enforcing this policy. Following a descriptive-analytic method and benefiting from library resources, the present paper aims to investigate this historical religious phenomenon and provide a response to the question of what reasons led to the decline of Shi’ism at the end of the reign of Őljeitű and his successors. The findings of this study indicate that Shi’ism became very strong during a specific time in Őljeitű’s period, and the Ilkhanate’s support played a significant role in reinforcing this religion. However, it could not continue its process of growth and was later marginalized.