With increasing applications of silicon carbide (SiC) power MOSFETs, more attention is being paid to reliability issues, among which the long-term stability of the gate threshold voltage is of paramount importance. In this study, laboratory experiments are conducted to investigate the threshold voltage instability under AC gate stresses with different duty ratios, and on & off-state gate voltages. It is found that no prominent drift would occur even for bipolar gate stresses, as long as the off-state gate voltage is within (higher than) the critical negative bias voltage which is related to the device fabrication process. Furthermore, not only the gate voltage swing but also the gate voltage polarity will affect the speed of threshold voltage drift when it occurs. The findings are intended for better understanding and management of potential threshold voltage drift in device applications.