The main question of this paper is: what are the epistemic functions of PNC (Principle of non-contradiction) in Aristotle's philosophy? To appreciate the epistemic functions of PNC we must be clear, first, about its meaning. Although Aristotle has stated its meaning in different ways, we can classify them into three different formulations; (a) logical; those expressions which - consists of concepts such as "statement" and "assertions", (b) ontological; the expressions which refers to PNC as a principle about "thing" or "being", and (c) psychological; that expressions which include the. Concept of "believe". According to him it is the ontological formulation which is fundamental formulation of PNC. This principle has characteristics as being non-hypothetical, improvable, the most certain, not being deceivable and being the best known of all. Such a principle with these characteristics has three important epistemic function some of which has been ignored even by Aristotle's famous interpreters. These functions are justification dependency which comes in relation to the role of PNC in justifying other beliefs, intentionality dependency, which comes from the role of PNC in relation to the meaningfulness of any word, whatsoever, as well as Practical dependency; which according to Aristotle any action requires the acceptance of PNC. It means that no one can do anything without accepting PNC