This paper presents a practical approach to accurately measure the in-circuit power loss of an integrated power electronics module. Based on several reasonable assumptions, the heat flux out of the module is related to the temperature drop across the thermal resistance material in the heat dissipation path. By performing a calibration experiment, the total dissipated heat can be identified. The experimental results prove the high accuracy of this measurement method. This measurement method can be extended to other situations when the assumptions that are applicable to this experiment are satisfied.