In this study, we investigated the thermal stability of Ni/Ag-based alloy contacts on p-type GaN. We observed the morphology of aggregated Ag for the Ni/Ag bilayer on p-type GaN after annealing it at 500 degrees C in an O-2 ambient. To improve the thermal stability, we deposited a Ni/Ag/Au trilayer onto p-type GaN. In this case, Ag aggregation was retarded after thermal annealing, and the specific contact resistance exhibited improved stability. Furthermore, because strong interdiffusion of the Au and Ag layers leads to poor reflectance, we added a diffusion barrier layer into the system; i.e., we deposited Ni/Ag/Ti/Au onto p-type GaN, with the Ti layer playing the role of the diffusion barrier. After annealing, the contact exhibited diminished Ag aggregation and a lower level of interdiffusion of the Au and Ag layers. We investigated the effect of the annealing time (at 500 degrees C in an O-2 ambient) on the properties of the Ni/Ag (1/150 nm), Ni/Ag/Au (1/150/150 nm), and Ni/Ag/Ti/Au (1/150/500/150 nm) layers, namely, their values of specific contact resistance, determined using a modified transmission line model, and reflectance at 465 nm. According to analyses by using scanning electron microscopy and secondary-ion mass spectrometry, we determined that the aggregation of Ag and the interdiffusion of Au and Ag within the Ni/Ag/Ti/Au construct were both minimized with the presence of the Ti layer, thereby improving the thermal stability of the contact on the p-type GaN.