This work investigates the effect of post oxide annealing (POA) on the AlGaN/GaN MOS-HEMT with a gate oxide grown by the H2O2 oxidation technique. The experimental results suggest that the gate dielectric layer with the POA treatment improves the dielectric material quality and MOS-HEMT performances. The dielectric breakdown field (EBD) of Al2O3 is improved from 6.5 to 7.9 MV/cm. In addition, the hysteresis measurement suggests that the fixed charges in Al2O3 and the interface charges (Dit) at Al2O3/AlGaN interface are reduced. This work also finds that the POA affects polarization charge density and improves MOS-HEMT performances.