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 ~17.6, 7.4 × 1011 eV−1·cm−2, and 4.6 × 10−8A/cm2 (at −2 V), respectively, showing potential application to the GaN-based metal-oxide-semiconductor capacitor.