Traditionally, it has been argued that increased economic competition through free trade agreements would bring about environmental degradation. This study, however, argues that recent international free trade agreements have tended to enhance environmental cooperation among participating countries. This study has examined the process by which East Asian countries have developed mechanisms for the extant level of regional environmental cooperation, particularly highlighting the reasons for commonalities and differences in regional environmental cooperation between ASEAN and the dominant economies in the region. It finds that three factors particularly matter for developing regional environmental cooperative mechanisms: networks of intergovernmental organizations, the strong willingness of political leaders which is often embodied in national strategies for regionalism and the establishment, and the institutionalized linkage-particularly through FTAs-between trade and the environment. Tracing the process of policy evolution within three groups of countries sheds light on the political conditions under which the four entities involved (the ASEAN, Japan, China, and the Republic of Korea) have produced and strengthened cooperative environmental mechanisms among them along with free trade agreements. Focusing especially on the environmental policy changes in Japan, China, and the ROK associated with the creation of its FTAs with ASEAN, the study concludes that each of free trade agreements has incrementally developed environmental cooperation, especially when integrated into a vision for regional integration.