In the study, a self-made kW-class 40-cell fuel cell stack with an active area of 112.85 cm2 was tested. The tests of the membrane resistance, charge transfer resistance, and the mass transfer resistance at different stack temperatures and gas humidity levels were performed. The results of the electrochemical impedance spectroscopy tests show that the temperature difference between the stack temperature and the dew point temperature has a significant effect. The fuel cell stack has a better performance when the dew point temperature is lower than the stack temperature. Even at low temperatures, it is recommended to maintain the stack temperature higher than the dew point to obtain a better performance of water management. It is concluded that the best temperature difference is between 5 ° C to 10 ° C. Moreover, the gas diffusion is the major promoting to the mass transfer resistance. Therefore, the suitable temperature difference is favorable for the gas diffusion and gas concentration in the gas diffusion layer.