Abstract: The heat from the lunar base was not rejected because there was a large temperature range on lunar surface, and a concept of heat pump used to reject the heat was proposed. A optimal study which can lighten the system mass was performed by qualifying the effects of several parameters such as the refrigerants, heat rejection temperature, cooling load, power supply specific mass, and radiator specific mass. The results indicate that the refrigerants whose critical temperature is high, such as R11 and R717, should be chosen since lunar surface temperature is 400K in daytime. An optimal system mass and heat rejection temperature is calculated when the cooling load is a constant. Meanwhile, the heat system mass is larger as the cooling load increases. Lunar base using a heat pump system can enhance the heat rejection and reduce the total mass of the thermal control.