The purpose of this article is to investigate the role of the Mongol Il-khanate in the spread of Buddhism in Iran. The centre of this government was located in the north-western region of Iran, and traces of the Mongol Il-khanate have remained in this region. The article traces Buddhist cave temples in the north-western region of Iran, viz., Maragheh and Zanjan. It offers reports on the sites of Imamzadeh Ma‘sum, Pirhashem, Kohul Machid and Qirkh Kohul in Maragheh and Dashkasan Temple in Zanjan. Then, references to Buddhism in the historical sources of the Il-khanate period, have been traced. Buddhist cave-cut structures were abandoned or altered for other use, after the acceptance of Islam by the Il-khanate rulers, beginning with Ghazan Khan.