Multi-energy retailer (MER) is a new type of retailer participating in both the electricity and gas markets. In a cooperative and competitive market environment, how to find the optimal coalition structure (OCS) model for MERs is an unsolved problem. Thus, this paper proposes an OCS model for MERs in electricity and gas markets. First, an MER optimization model is built to maximize profit, and the market-clearing constraints are considered. A coalition game model for MERs including coalition value and OCS model is introduced, which describes the behavior of forming a coalition among MERs considering coalition cost and scale limitation. Moreover, a hybrid algorithm including the diagonalization algorithm (DGA) and dynamic programming algorithm (DPA) is designed to solve the OCS model. Finally, the case studies are performed on a system with IEEE 14-bus electric network and 7-bus gas network. The numerical results show that the proposed model can effectively obtain OCS.