Mu’tazilah is one of the sections which attach much importance to reason in the Islamic teology. Owing to the rationlist approchs, they have undertaken an important mission both in muslim society and against the non-muslims. Within the context of their five principles they attempted to give a form to the Islamic community. In this article, the effects of Mu’tazilah on the Islamic thought and their contributions to the Muslim community have been studied.