The surface of p-type GaN was treated with HCl, H3PO4, KOH, and aqua regia solutions before the deposition of Pd metal, and the ohmic contact formation mechanism was studied by observing the change of ohmic characteristics with the pretreatment of the surface. Contact resistivity decreased in sequence with treatment using HCl, H3PO4, KOH, and aqua regia solutions. The contact resistivity on p-type GaN treated with boiling aqua regia was decreased by two orders of magnitude in comparison with the HCl treated one. The amounts of oxygen and carbon atoms on p-type GaN were reduced with the same sequence of surface treatment and the aqua regia solution is most effective in selectively removing surface oxides without etching the p-type GaN. This provides evidence that the reduction of contact resistivity originates From the selective removal of surface oxides, acting as an impeding barrier for hole injection from metal to p-type GaN. Contact resistivities did not change when the hole concentration was varied from 2.8 x 10(16) to 2.5 x 10(17) cm(-3) in both the HCl treated and aqua regia treated samples. This suggests that the formation of ohmic contact on p-type GaN can be explained by the thermionic emission of holes at the interface of Pd with p-type GaN. The Schottky barrier height deduced from both electron affinity of GaN and work function of Pd is much differ from the value of about 0.35 to 0.45 eV experimentally determined using contact resistivities. This proposes that the Fermi level pinning at acceptor-type point defects on the surface of p-type GaN lead to the ohmic contact on p-type GaN.