Gate oxide degradation under dynamic gate stress has been demonstrated as a reliability issue for SiC mosfets recently. Investigating the influence of dynamic drain-source voltage stress (V<inf>DS</inf>) and load current (I<inf>L</inf>) involved in switching operation on gate oxide degradation is very significant to identify the way for effectively assessing gate oxide reliability. In this article, a buck converter with continuous switching condition and constant high temperature is built and operated to evaluate gate oxide degradation. Moreover, the results from buck converter are compared to results regarding ac bias temperature instability (BTI) under the same conditions for devices with different gate structures. The degradation degree of different gate oxide locations under the two operations is analyzed combining I-V and split C-V characteristics. It is found that there is consistent degradation of the gate oxide above JFET region, but depending on the operation mode, the degradation is different above channel region, indicating that V<inf>DS</inf> and I<inf>L</inf> have different effects on different gate oxide locations. Therefore, ac BTI test cannot sufficiently evaluate gate oxide degradation and may overestimate or underestimate its reliability, depending on the device structure and fabrication process. It is necessary to investigate the gate oxide reliability in typical switching operation.