It is misfortune that David Mitrany's functionalism is often simplified to be a theory of regional integration by some peoples without probing into his core of ideas. The purposes of this article are to introduce the content, goal and methodology of functionalism, and to review some critics with bias made by some scholars. Besides, we can know functionalism deeper by examining Mitrany's position in two Great Debates of International Relations Theory. The functionalism will be compared with Wilson's idealism, Carr's realism and Hass's neo-functionalism. Finally, this article will evaluate the contribution of functionalism in today's study of international relations.