In this work, the interrelation between the thickness of Ni metal layer and the characteristics of p-GaN/Ni/Au ohmic contact was experimentally studied. The behaviors of Ni/Au double-layer metal electrode in alloyed annealing process were discussed by XRD and metallographic methods. It is revealed that, due to the mechanisms of metal inter-diffuse and Ni-O oxidation, the ratio of Ni/Au layer thickness plays an important role in determining the characteristic of p-GaN/Ni/Au ohmic contact. According to the results, the best p-GaN/Ni/Au ohmic contact can be attained when the thickness of the double metal layers is equal approximately.