As a supreme body of Multilateral Environmental Agreements(MEAs), Conference of the Parties(COP) has played a key roles in implementing of MEAs. While COP decisions can be one of internal laws in terms of traditional international laws, COP decisions affect rights and obligation of parties through providing substance and enforceable obligation to the parties. When the traditional criteria of legal binding forces are applied to COP decisions, COP decisions are documents not entailing binding consequence. In other words, according to the traditional opinion, COP decisions are considered as gentlemen’s agreements or soft laws which have no legal binding force. However, this paper argues additional criteria, which are appropriate to the MEAs as special regime in terms of autonomous institutional arrangement, self-contained regime and quasi-international organization, shall be applied in consideration of the legal binding force of COP decision such as decision making process and decision making authority which provided in the MEAs. When the additional criteria of legal binding forces are applied to COP decision, some of COP decisions which are adopted in consensus and are adopted in majority rules with explicit authority are entailed to binding consequence. Consequently, some parties not voting for the COP decision are also subject to be enforced by the COP decision. This is abnormal to the state voluntarism based on the sovereignty doctrine. However, the unique role of COP and COP decision in special regime of the MEAs for international cooperation shall be considered in order to understand currents practice of COP decision in MEAs.
