Rationalism and its effect in religious studies have a long background among Shiites` commentators. Sheikh Tusi may be regarded as one of the precedents in the field. The paper, describing reason and rational approach in commentary and relying on two verses of the Quran, has studied the grounds and appearance of the approach in commentary. It is clear that, when taking such an approach, a commentator would bring some norms into consideration. In this study, the most notable and dominant particularities have been referred to at first and then, the verses 83 and 149 of An`am with regard to some examples have been studied. what discovered in the study lies in national norms in the commentary of tebyan. Reasonability along with opposing imitation in religious studies, hermeneutics as an outstanding norm of rationalism in commentary, reasonable critique of traditions, ijtihad and theological inferences and giving various semantical modes of meaning for the verses and literal analysis may e regarded as Tusi`s particularities of rationalism.