The structural and electrical characteristics of a thermally grown gallium oxide on GaN films were investigated. A furnace oxidation at 850 degreesC for 12 h resulted in the formation of monoclinic beta -Ga2O3, 88 nm in thickness. From the I-V measurements of the metal-oxide-semiconductor (MOS) structure, the breakdown field strength (E-BD) was found to be 3.85 +/-0.32 MV cm(-1), Under gate voltage sweep, C-V hysteresis was observed due to the oxide charge trap. By comparing the ideal and experimental C-V curves, the oxide charge density (NI) was calculated to be 6.77 x 10(11) cm(-2). These results suggest that the thermally grown Ga2O3 is suitable for gate dielectric applications of power MOS field-effect transistors.