It is hard to reject that Fakhruddin al-Razi is considered as a brilliant figure who has deep knowledge in many sciences, not least in the field of Ushul Fiqh. This article is in attempt to describe his most important and monumental work in ushul fiqh study, al-Mahshul fi Ilm al-Usul. The book written in the seventh century Hijri is also the important work in the discipline of usul fiqh, due to its comprehensive discuss in the subjects of ushul fiqh. It is also regarded as the best work in this field, especially among mutakallimin