Mg2TiO4/MgO composite thin films were deposited on GaN (001) as gate oxides. The use of oxygen in sputtering and post-annealing increased the (111)-preferring orientation of Mg2TiO4 on GaN (001). Inserting MgO buffer layer slightly decreased the overall dielectric constant, but considerably improved the electrical properties by modifying the band alignment at interface. The relative permittivity, interfacial trap density, and leakage current of the composite layer-based capacitor were similar to 17.6, 7.4 similar to x similar to 1011 similar to eV-1 center dot cm-2, and 4.6 similar to x similar to 10-8A/cm2 (at -2 similar to V), respectively, showing potential application to the GaN-based metal-oxide-semiconductor capacitor.