This MA thesis traces how “ethnic Koreans” in northeastern China (chaoxianzu) reshaped their perception(s) of “ethnicity” over the course of the great political and social upheavals from Manchukuo to the People’s Republic of China. By looking into less-explored memoirs and oral histories, this research is interested in dissecting the interrelations between memory-formation and ethnic imagination. Chapter 1 lays the theoretical groundwork for my memory-centered approach, through which I historicize the ethnic Koreans’ conceptualization(s) of “ethnicity” as a process, rather than a self-evident precondition. Chapter 2 reveals the ethnic Koreans’ ambiguous and fluid sense of ethnicity under Manchukuo’s ideology of minzu xiehe (concordia of ethnos). Chapter 3 examines the cultural construction of “Korean ethnicity” advocated by the Chinese Communist Party during the Chinese civil war. Chapter 4 investigates the contestations between the Party-state’s revolutionary narrative and the bottom-up ethnic discourse in the early socialist era. This thesis argues that memory comes to be a mediator reifying the fluid, contingent, and sometimes-contested process of ethnic imagination in between the boundaries of nation-states.