We have investigated the ohmic contact formation mechanism of Pd-based ohmic contacts to Mg-doped p-GaN grown by metalorganic chemical-vapor deposition. To examine room-temperature ohmic behavior, various metal contact systems were deposited and current-voltage measurements were carried out. In spite of the large theoretical Schottky barrier height between Pd and p-GaN, Pd-based contacts showed perfect ohmic characteristic even before annealing. According to the results of synchrotron x-ray radiation, the Pd (111) layer grew epitaxially on GaN (0001) surfaces with Pd[1 (1) over bar0]parallel to GaN[11 (2) over bar0]. The closed-packed atomic planes 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 0.47 eV, which was estimated by the Norde method. This discrepancy between the theoretical barrier height and the measured one may be due to the epitaxial growth of Pd contact metal.