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The authors report improved electrical properties of nonalloyed Ti/Al ohmic contact to N-face n-type GaN via oxygen (O2) plasma treatment. The contact resistivity of Ti (50 nm)/ Al (35 nm) electrodes is reduced significantly from 4.3 × 10-1 Ω · cm2 to 2.53 × 10-5 Ω · cm2 by applying O2 plasma to the GaN surface before the Ti/Al deposition. In this process, Ti-N bonds are expected to form, while the Ga-N bonds are broken by the O2 plasma, which eventually increases the nitrogen vacancies as well as the Ti-N phases at the GaN surface. This suggestion has been verified by X-ray photoelectron spectroscopy and transmission electron microscopy analyses.