A method of protecting a power device of an inverter from overheating when a motor stalls or is operated in a low-speed range, including detecting and then performing an operation on a maximum tolerable temperature at a junction of the power device of the inverter and a casing temperature between the power device and a radiating plate, calculating an absolute value of an operating speed, and applying a pattern gain differently depending on the calculated absolute value, calculating inverter loss resulting from input motor torque and speed of the motor, performing an operation on the values calculated and calculating a difference between the temperature at the junction of the power device of the inverter and the casing temperature, limiting output of a PI controller that receives the temperature difference calculated, and limiting operational torque output of the motor according to the input motor torque command using the output of the PI controller.