After the late 1990s, moves towards regional integration and cooperation gained momentum in East Asia. The regional countries have expanded and deepened integration initiatives under the ASEAN Plus Three (APT) framework that comprised of ASEAN countries, China, Japan and South Korea. What factors have promoted the development of regional integration and economic cooperation in the region? This article addresses this question in terms of collectively shared norms and political leadership. Informality, a representative common norm, played a catalytic role in inducing a reluctant state to join the cooperative framework and mitigating opposition from countries outside the region. Importantly, the development of regional cooperation under the APT framework was sustained by the shift of the policymakers’ preferences from the informal to formal settings. Moreover, political leadership shown by China and Japan has played a crucial role in promoting the regional integration initiatives. Japan has taken the lead in developing financial and monetary architectures and other cooperative mechanisms, while China has taken the initiative in propelling regional free trade agreements and economic development and integration in the Indochina countries. Rivalry for political leadership between the two countries has induced them to provide regional public goods in the positive-sum game manner.