This paper tries to analyze how to define the boundary of eastern Taiwan as a research object. Moreover, this paper also to discuss the social contexts and social meanings behind the different definitions for the boundaries of eastern Taiwan. Basically, the area of eastern Taiwan is Kind of historical space, and to be Kind of contemporary social construction as well. In addition, boundary always means the difference between two sides, defining the boundary of eastern Taiwan eventurally means the difference between eastern Taiwan and others. This paper argues that the boundary of eastern Taiwan was defined by historical context, state government and geographical imagination, Acturally, it is transitional, expedient, and multi-meaningful.