This paper investigates the failure diagnosis of the brushless DC motor with a small level of the rotor magnet defect by using the developed IPMSMs and the measurement system. The Fourier analyses of stator voltage and stator current and their amplitudes are available to show the difference of the magnetization of permanent magnets in spite of very small demagnetization or imperfect magnetization. It is also shown that the continuous Morlet and Paul Wavelet analyses of stator voltage and stator current at a speed step response are available to show the difference of the magnetization of permanent magnets.