The best way to understand any scientific discipline is to determine its composition correctly. The work for this purpose was in the form of identifying its definition and subject. Usul al-fiqh, has been the subject of such studies since the earliest days and different approaches have been put by Islamic scholars forward to determine its definition and subject. It can be said that the different approach to the subject which adopted by Sadr al-Sharia (d. 747/1346) who is one of the Hanafi jurists, has an important place in the usul al-fiqh tradition. Sadr al-Sharia has defined fiqh and usul al-fiqh differently from tradition and has adopted an original perspective to the determination the subject of the usul al-fiqh. In this article, we will try to determine the Sadr al-Sharia's critical approach of concerning the composition of the usul al-fiqh and its reflections of this approach in tradition.