In spite of few diversities in its extent and boundaries, infallibility of prophets is a common belief of most numerous of the Muslim theology sects and many of other revealed religions, believers. Therefore, it has a special situation and importance among theological subjects.Among Shiite thinkers, there also are great theologians both ancient, like Sheikh Mofid and recent like AllamehTabatabaii have turned to various features of this object. AllamehTabatabaii being bound over rational proofs, regards immunity from oversight, forgetfulness, and mistake in reception, communication, propagation, and description of revelation as necessities of prophetess, and defines immunity from sin as keeping away of duty abandonment and doing prohibations, namely actions opposed to servituale and God’s mastership. He regards the negation of mistake in palpable perception and nominal sciences, and recognition of expedience and corruption of creative affairs out of the object of "prophets in fallibility", so, adduse no reason about the matter.Sheikh Mofid in his turn, regarding the infallibility from Holy prophet superior to another ones, believes His Excellency to have infallibility from any kind of purposeless and purposeful, cardinal or lesser sin, from birth, and considers other prophets liable to purposeless lesser sins that don’t cause the doers to be despised, before their prophetic mission.The present article tends to explain and comparer the opinions and reasons of these two thinker about “prophet's infallibility", based on library method and text analysis. The most important result of this article is clarifying the similarity of viewpoints of these two grand theologians about absolute infallibility of all prophets in the area of revelation and prophecy, during their prophetic missions, and their diversity of thoughts, out of the prophecy domain before their prophet hood.