The purposes of this comparative study are to inquire into the backgrounds, processes, and political effects of the electoral reforms of Korea and Japan. More specifically, this study examines why and how Korea and Japan reformed their electoral systems in 2004 and 1996. In addition, this study analyzes what new electoral systems in Korea and Japan influenced in the electoral processes and outcomes. Finally, this study evaluates the effects of the electoral reforms and proposes the additional institutional arrangements for the political developments in Korea and Japan.