We investigated the nano-structural evolution of metal contacts to GaN during annealing and the correlations between nano-structures and electrical properties for two typical ohmic contacts; Pd as a non-alloyed contact and Ni/Au as an alloyed one. Pd was completely transformed to two kinds of epitaxial Pd gallides, Ga2 Pd5 and Ga5 Pd, at 700 ºC. In the alloyed Ni/Au contact, the reaction chemistry was rather complicated. Ni4 N formation was found in N2 annealed sample at 500 ºC of which reaction kinetics was greatly affected by the catalytic effect of Au. The hightemperature compounds were correlated with the rapid degradation of electrical properties during annealing. Meanwhile, the thermal stability of Ni/Au contact greatly improved by suppressing the activated Ni reactivity, which was able to be obtained by forming preferential Ni-O bonding through annealing in air.