Usul al-fiqh as a unity of source and method rules is characterized by the jurist’s (mujtahid) personal preference. Usul al-fiqh, the highest-level Islamic science, was constituted by the common contribution of Sunnite and Mu’tazilite jurists. Therefore, the attribution of usul al-fiqh to Ahl al-Sunnah or al-Mu’tazilah is not true except being based on the distinctive qualities belonging to each school. The consideration of shar’iyyah is a distinctive quality or essential feature for both Ahl al-Sunnah and al-Mu’tazilah. In this article, it has been examined the concept of shar’iyyah as a criterion in the distinction between the Sunnite and Mu’tazilite usul al-fiqh.