With economic development and trade liberalization becoming the main engines of international relations in the post Cold War period, a new wave of regionalism has also arisen. In contrast to Southeast Asia’s early move in establishing free trade (ASEAN) in 1992, China, Japan and Korea’s interest towards regional cooperation remain limited, which has greatly hindered the progress of integration in turn. As an economic power in East Asia, Japan plays a critical role in regional integration. Through an examination of Japan’s actions towards regional cooperation in the post Cold War period, perhaps we could begin to understand the rather slow development of East Asia regional integration. This article seeks to examine Japan’s response towards external and internal developments and analyze transformations in Japan’s regional cooperation towards Northeast Asia from the perspective of neo-classic realism. This article first points out the key concepts of neo-classical realism and then re-examines Japan’s policy transformation in order to gain insights and understanding on the development of East Asia regional integration.