It is very difficult to measure and classify accurately the mechanical losses in the internal combustion engine running under power. In this paper, the authors present the retardation method containing two graphical approaches derived from the differential equation of motion under free revolution, cutting off the power, to evaluate and classify the mechanical losses in the engine, and then compare this method with the conventional methods commonly used. The experiments obtained by the retardation method show high accuracy of the evaluation and discrimination of mechanical losses in the engine. In conclusion, the retardation method is proved to be a useful technique for the measurement of mechanical losses in the engine, and supposed to be better than the methods commonly used since it can classify the each component of mechanical losses.