We have investigated surface treatment effect on the interfacial reaction of Pd/p-GaN interface and also room temperature ohmic contact formation mechanism of Pd-based ohmic contact. In order to examine room temperature ohmic behavior, various metal contact systems were deposited and current-voltage measurements were carried out. In spite of large theoretical Schottky barrier height between Pd and p-GaN, Pd-based contact showed perfect ohmic characteristic even before annealing. According to the results of synchrotron X-ray radiation, the closed-packed atomic planes (111) of the Pd film were quite well ordered in surface normal direction as well as in the in-plane direction. The effective Schottky barrier height of Au/Pd/Mg/Pd/p-GaN was 0.47eV, which was estimated by Norde method. This discrepancy between theoretical barrier height and the measured one might be due to the epitaxial growth of Pd contact metal and so the room-temperature ohmic characteristic of Pd-based ohmic contact was related strongly to the in-plane epitaxial quality of metal on p-GaN.