This paper presents a new thermal protection approach for permanent magnet synchronous motor. Usually the thermal protection model is built based on two parameters which are thermal resistance and thermal capacitance of motor. But the stator core loss is increased significantly when the motor frequency and the motor winding temperature are increased. If the motor runs at high speed, the eddy current loss, the hysteresis loss and the harmonic current loss dominate the stator core loss. If only the thermal resistance and thermal capacitance are used for the thermal protection without considering the motor frequency, the protection model cannot response the temperature rising when the motor runs at high speed, so the motor could not be protected effectively. In this paper, the motor frequency is included in the new thermal protection approach, and the whole working speed range will be protected, and the motor lifetime will be increased.