GaN metal-oxide-semiconductor (MOS) capacitors were fabricated by using Ga oxide formed by photoelectrochemical oxidation of GaN. The electrical properties of the MOS structures as characterized by capacitance-voltage measurement were found to be dependent on the oxidation time and posttreatment. Positive flatband voltage was observed in devices with thin oxide layers indicating the existence of negative oxide charge. Very thin oxide exhibits high capacitance and reverse leakage, which can be reduced by rapid thermal annealing (RTA). Passivation of the interface by RTA is partially responsible for the improvement. Thicker oxide layers exhibit improved electrical properties. Low density of interface states (1011eV21cm22) was obtained in the Ga-oxide/GaN structure grown under optimized conditions.