We reveal the existence of interfacial, epitaxial, Pd grains in an as-deposited Pd film evaporated on GaN(0001) at room temperature. The crystallization of the remnant, disordered Pd was caused by the growth of the interfacial grains during annealing up to 600 degrees C. The origin of the Pd epitaxy on GaN( 0001) was attributed to a 6/7 matched interface structure, wherein 6-Ga atomic distances match 7-Pd atomic distances. Remarkably, the Pd film was, by the Pd-Ga reaction, completely transformed to Ga2Pd5 and Ga5Pd gallides in epitaxial relationships with GaN at higher a temperature of 700 degrees C.