Based on the law of energy conservation, a power flow model of high-speed motorized spindles was presented. Quantitative power loss models of the built- in motor, bearings and air were developed. An experiment on the 2ZDG60 motorized spindle was carried out under several operating conditions. The good agreement between the theoretical results and the experimental data indicated that the power flow model was capable of accurately predicting the power loss of high-speed motorized spindles. And on all these points, the thermal simulation data of 2ZDG60 motorized spindle showed that the temperature rose on the areas of bearings and motor was higher. The little error of testing data and simulation data of temperature rise proved the correctness of the power flow model.