An original-designed thermal control scheme that manages the thermal behaviors in a proton exchange membrane (PEM) fuel cell generator has been proposed. It not only keeps the stack from overheating under extreme high external loads, but also prevents the stack from staying too cold in the cold-start conditions. A thermal control unit (TCU) together with a smart control algorithm is able to limit the fuel cell operation temperature in a desired range. The TCU comprises mainly a thermostat, a radiator, and a heater. It divides the stack coolant into a cooling stream and a heating stream that maintains a pre-set coolant temperature before entering the stack. Parametric studies include the external loads (0< PL <4kW) and the stack coolant inlet temperature (SCIT=53, 58, and 63°C). The dynamics of SCIT under different loads are measured to verify the thermal reliability of the fuel cell generator. Then, examining the effect of SCIT on the system efficiency assesses the performance the fuel cell generator. Finally, an empirical correlation for the system efficiency of the PEM fuel cell generator under different SCITs is presented as a function of the external loads.