Abstract: The Middle Ages is the important period in the development of logic, and the theory of supposition is the creative and distinctive theory in medieval logic. The theory of supposition studies the relationship between the terms in propositions and their reference and it provides a unique approach to analysis of language. Ockham's theory of supposition was typical and representative in that period. It shows that not only the subject but also the predicate in a categorical proposition have the property of supposition,and supposition plays an important role in judging the truth of the various propositions including singular propositions, particular propositions, universal propositions, indefinite propositions, past-tense and future-tense propositions and modal propositions,in order to show that supposition plays a basic role in judging the truth of the proposition.