This paper presents the optimization of an onboard system that uses a single mixed refrigerant (SMR) to reliquefy boil-off gas (BOG) for liquefied natural gas (LNG) carriers, considering the partial load requirement according to the operational profile. The amount of excess BOG varies depending on fuel consumption, which is determined by the ship's speed. Therefore, the optimization must consider equipment characteristics at partial load as well as at design load. Three operational loads are defined for a 174 000 m(3) LNG carrier that uses an SMR reliquefaction system. Optimization to maximize net present value is performed using a genetic algorithm. The optimal result indicates an energy consumption of 678.1 kWh/(ton LNG) at design load can be achievable, at which the net present value is US$0.76 million for the given operating conditions.