The Ni/Pt and Ni/Au contacts to p-GaN heat treated in air are investigated to study the effect of the metallic phase of high work function on specific contact resistance (p c ). The current-voltage measurement suggests that there is an optimal microstructure for the oxidized Ni/Au contact to obtain a minimum p c of 4×10 -6 Ω cm 2 . Such low ohmic contact resistance could be related to the formation of the NiO and the specific microstructure. However, high p c of about 2-5 × 10 -2 Ω cm 2 are obtained for the oxidized Ni/Pt contacts, which is attributed to the different microstructure from that of the oxidized Ni/Pt contact. The NiO phase of the oxidized Ni/Pt contact is separated from p-GaN by a continuous face-centered-cubic Pt-Ni-Ga film. Therefore, the oxidized Ni/Pt contact behaves just like a common metal contact, but not a metal/NiO composite contact. Comparison of the two oxidized contacts implies that the previously proposed model based on metal/p-NiO/p-GaN heterojunction is reliable to explain the low p c to p-GaN.