Abstract: As a high degree of reliability is necessary to permanent magnet synchronous motors (PMSMs), the detection of a precursor to the demagnetization of permanent magnets (PMs) is very important. In this talk, we will investigate the failure diagnosis for PM demagnetization. Physical variables to be available for diagnosis will be clarified by simulations under controlled using a constant V/f strategy and a vector control strategy. The simulation results will be experimentally verified.