This study deals with the questions of "canon" and "canonicity" that have effected all fields of literary study, and examines canon formation in American poetry especially in the first half of the twentieth-century. Discussing the canons in modem American poetry, this paper introduces some canon debates and American poetry anthologies, explains the New Critics" implications for the modem American poetry canon, and emphasizes the fact that there is not a singular tradition of modern poetry.<BR> Recently the canon formation has become as "an arena of struggle" over questions of a more "systemic nature" than "the rise and fall of individual reputations." At universities, however, expanding the canon is not a viable solution; choice is mandatory. I suppose that whatever the motivation of canon selection, it remains important to contest the constrictive tendencies of the critical and pedagogical lists, because we need to be wary of intellectual stagnation. At the same time, ideologically oriented critics who often cite the relativity of "truth" must also reflect that their own social and political beliefs can be less relative than others. Considering the ongoing process of defining, and redefining the canon, this study draws a conclusion that the literary canon, not as a single authoritarian list but as a set of options, is relatively stable at one end and relatively open at the other. At the practical level, college teachers will always have competing canons. Thus "the multiplicity of possible purposes" remain substantially important.