Recent developments of multilateral Mechanisms in the Asia Pacific region have shown an increase in states’ confidence and their intention of making collective responses to common concerns of economic and security regionalization. Nevertheless, at the nascent stage, not all the regional powers, including the U.S. and China, were interested in taking a multilateral approach to regional order. In contrast, Japan showed strong interests in taking multilateral initiatives and became founders of both Asia Pacific Economic Cooperation (APEC) and ASEAN Regional Forum (ARF). This article explores the strategic rationales and impacts of Japan's initiations from the perspective of regional institution-building.