In this paper, a condition monitoring method for gate oxide degradation of trench silicon carbide (SiC) power MOSFET is presented. In this method, the body diode voltage at the inflection point of the output characteristic curve of the third quadrant SiC MOSFET at zero gate voltage is used as the characteristic parameter. The current monitoring methods of electrical parameters are often inaccurate due to coupling junction temperature, or cannot achieve on-line monitoring due to intrusiveness, but the proposed method can ignore the effect of junction temperature, and provides a good foundation for on-line condition monitoring.