In this paper, we investigate the effect of annealing conditions on the characteristic of Ni/Au Ohmic contact to p-GaN. The specific contact resistivities under different annealing temperature and different annealing atmosphere are tested using the circular transmission line model. It is found that the best annealing temperature is about 500 ℃. The annealing atmosphere of nitrogen-oxygen gas mixture can lead to lower specific contact resistivity than that of pure nitrogen, and the specific contact resistivity has no relationship with the content of oxygen. Finally, we obtain the lowest specific contact resistivity to be 7.65×10-4 Ω·cm2 at the best annealing temperature and atmosphere.