This paper highlights the co-management of an MPA in Trao Reef locally managed marine reserve, which was established in 2001 to protect and rehabilitate fisheries resources in general and the coral reef in particular. In addition, this paper demonstrates one way to approach comanagement which include the transaction-costs, the method for measuring the transaction costs in fisheries co-management system. Transaction costs are defined as “the cost of transacting, which consists of the costs of measuring the valuable attributes of what is being exchanged and the costs of protecting rights and policing and enforcing agreements” (North 1990). The study is based on fisheries management, co-management, transaction-cost literature and secondary and primary data. The reduction of transaction-costs in the last stage of comanagement regime is used to choose alternative institutional arrangements for managing a fishery for public policy decisions. This study is also the first paper to mention transaction-costs in fisheries co-management in Viet Nam.