The practice of multilateralism appears at the same time as the emergence of states Or community societies, such as, the vertical and horizontal alliance in the Warring States Period (75B.C.-221B.C.) of China . However, it is since the end of the First World War that multilateralism began to become a main practical activity in international relations, Then, after the Cold War, the theory of multilateralism was paid more attention and got further development, which is relative authoritatively defined as "an institutional form that coordinates relations among three or more state son the basis of generalized principles of conduct"(Ruggie, 1993, p11). Nowadays, multilateralism is widely embodied and applied in the area of economic, politics, military and security, etc. From a security perspective, although the end of the Cold War contributed to the ease of the antagonistic relationship, it brought uncertainties to the East Asia that today continue to trouble analysts and policymakers alike. Concerns include tensions in the Taiwan Straits, uncertainty on the Korean Peninsula, confrontations in the South China Sea, and hostility in South Asia, as well as ongoing territorial and maritime disputes, armed insurgencies, and ethnic strife (Acharya, 1995; Naidu,2000).Accordingly, at the same time, with the development of regional integration, regional security cooperation has made great progress, and multilateralism began to sprout and has penetrated into the international affairs in East Asia, such like the establishment of Asian Regional Forum(ARF), Council for Security Cooperation in the Asia Pacific,(CSCAP) and 6-party talks, However, it is fundamental change in the regional Security structure before and after the Cold War, which means the multilateral security Cooperation mechanisms operate ineffective governance, and the peace is still maintained by the bilateralism, particularly the bilateral alliances established during the cold war, for example, the U.S-Japan alliance, the U.S- South Korea alliance, the Sino-North Korea alliance, the U.S, Philippines alliance, etc., in other words, the security cooperation is hovering between the multilateralism and bilateralism. All in all, currently, multilateralism is budding and developing in security cooperation, while mass of adverse factors and obstacles ahead. The thesis will review the existence and prospect of the multilateral security Cooperation in the East Asian region, and specifically focus on what causes the multilateralism's ineffectiveness application to the security cooperation in East Asia. I will first explain three interpretations of multilateralism, followed with proffering three hypotheses, and then two cases of regional security structure in East Asia, and finally find the key independent variables responsible for the current stalemate in East Asian multilateral security cooperation. In the literature review part, three interpretations of multilateralism will be stated: the multilateralism as a strategy, the multilateralism as an institutional structure, and the multilateralism as a value. Firstly, "Multilateralism as a strategy" is an interpretation from the perspective of individual state, which means a state prefers the multilateral diplomacy to deal with its external affairs,. This sense of multilateralism can be seen as a strategy for a state's external representation, like isolationism, imperialism, he Non-Aligned strategy, etc. The second interpretation is "regarding multilateralism as an institutional structure". Different from the first interpretation which analyzes at the level of individual state, this perspective is at the level of international system. Under this interpretation, multilateralism refers to a type of international interaction, It is looking from a global or regional perspective and emphasizes the functions of multilateral institutional structure, especially, how institutional factors affect the interaction among states., Thirdly, from the view of constructivism, multilateralism could not be inseparable with norms, beliefs, and group identity, and an ideology "designed" to promote multilateral activity. According to the three interpretations of multilateralism, this thesis establishes three hypotheses and tests them with two cases study by applying realism, neo-liberal institutionalism and constructivism theories to find the independent variables responsible for the current stalemate in East Asian multilateral cooperation. The hypotheses are: 1) Power conflicts hinder security cooperation in East Asia. 2)Loose and low institutionalization hinders security cooperation in East Asia; 3)Differences in identities hinder multilateralism in East Asia. Besides, the multilateral resolution to South China Sea issue from Southeast Asia and multilateral resolution to the North Korea nuclear issue from Northeast Asia are two cases. Consequently, the first two hypotheses turned out to be correct.