A systematic study of discrete SiC MOSFETs' reliability under High Temperature stress has been carried out. High Temperature stress is performed in this work to characterize the threshold voltage instability. To investigate the degradation mechanism of devices, simulation according to the structure of MOSFET cell has been performed. The result shows that the threshold voltages change trends of both MOSFETs are the same, including drop-down period at very early time due to a decrease of doping concentration at channel region and gradual raise-up period at in the rest of time resulting from decline of interface charge.