This paper analyzes the characteristics of the brushless DC motor with permanent magnet defect for failure diagnosis. We have manufactured an interior permanent magnet motor, in which one of four magnets is reduced by 10% and 20% in order to imitate the demagnetization. This paper clarifies the characteristics of the developed motor, and shows that the Fourier analysis of the amplitude of stator current and stator voltage is useful to show the difference of the demagnetization of permanent magnets.