The effect of Au overlayer on Ni contacts to p-type GaN was investigated by comparing the change of electrical properties with interfacial reactions at the interface of Ni/p-type GaN. The contact resistivity at the interface of Ni/p-type GaN was decreased by two orders of magnitude at 600 degrees C by incorporating the Au overlayer on the Ni contact. This originates from the outdiffusion of Ga atoms from p-type GaN to the Au overlayer, and subsequently produces Ga vacancies acting as accepters for electrons below the contact. Consequently, net hole concentration increases below the contacts, resulting in the reduction of the Schottky barrier height for hole transport, via the decrease of contact resistivity.