Reverse bias annealing (RBA) is applied to N-polar GaN high electron mobility transistors (HEMTs) to improve the quality of the gate stack interface. As demonstrated for Ga-polar HEMTs, RBA improves the stability of the gate stack interface. However, the decrease in the maximum drain current density is observed as a unique phenomenon for the N-polar HEMTs. The calculation of the band profile suggests that in the N-polar HEMTs the electrons injected from the gate electrode by the reverse bias accumulate at the gate stack interface in the extrinsic gate region. This promotes the electron trapping in the gate stack, which results in an increase in the source access resistance by the virtual gate phenomenon. In the Ga-polar HEMTs, the electrons tend to accumulate at the AlGaN/GaN interface rather than the gate stack interface, which gives less chance of the virtual gate phenomenon.