In recent centuries, orientalists, in general, and Western Mahdiologists, in particular, have brought more attention to the topics of "Mahdi" and "Mahdism".  A considerable number of Western thinkers, relying on historical studies, have reviewed the studies on Mahdi and Mahdism researches. By examining the Shiite doctrine of Mahdi and Mahdism, Bernard Lewis studies the process of believing in Mahdi and its related social actions. Though little is known about Lewis' Mahdist researches in the available sources, this paper through inquiring and according to a descriptive-analytic method paraphrases his Mahdavi studies. The most important claims of Lewis about Shiite Mahdism are such items as invalidity of the belief in Mahdi and Mahdism, being influenced by the idea of Saviorism in Zoroastrian and even Judaism and Christianity, taking advantage of this idea to mobilize people in Shiite society against the enemies and colonial powers