Abstract: The reasonable charge volume is an important condition for normal operation of the pump-driven two-phase flow cooling system. In this paper a pump-driven two-phase flow cooling experimental system is built to study effect of the charge volume. The experimental results show that the maximum allowable charge volume of the system is affected by the heat load. It is necessary to keep in the system a gas-phase space which is not smaller than the steam volume caused by the heat load. When the heat load increases, the maximum charge volume of the system decreases. The reduced volume is the evaporation space, which is needed by the refrigerant to take away the new heat load. When the system charge volume exceeds the allowable value, the system work point will be deviated. That deviation will induce the phenomena of heat transfer deterioration and pressure rise.