The Mu’tazilah have been interested in Usul al-Fiqh (legal doctrine) since their founder Wasil b. ‘Ata (d.131/748). There are some books on Usul al-Fiqh belonging to some of the Mu’tazilah scholars. Rational, liberal, philosophical and thelogical opinions of the Mu’tazilah were reflected on their understanding of Usul al-Fiqh. The Mu’tazilah have composed Usûl al-Figh with the mutakallim’s method that based on rational evidence. Therefore the discipline of Usul al-Fiqh has gained depthness. The Mu’tazilah have influenced some of Shii and Sunni scholars in the field of Usûl al-Fiqh.