The influences of the process conditions on Ag/p-GaN ohmic contact have been studied. An optimized process was obtained and applied in the fabrication of devices. Ag patterns were prepared by three kinds of processes: direct lift-off, lift-off after an oxygen plasma treatment, and wet etching. The adhesive and electrical properties of the samples which were fabricated by the three kinds of processes were compared. It was found that the Ag layer is easy to peel off after direct lift-off process, and the ohmic contact could not be formed after an oxygen plasma treatment. In contrast, the wet-etch samples showed better adhesion and electrical properties after annealing. The mechanism of the influences of different processes on the contact performances were analyzed by X-ray photoelectron spectroscopy. Furtherly, surface chemical treatments before Ag deposition were compared and optimized. The results showed that acidic or alkaline solution treatment could effectively reduce ohmic contact resistivity, and the effect of acidic solution treatment was slightly better. The optimized ohmic contact process can be applied to fabricate the visible light and deep ultraviolet (DUV) LED devices. At 40A/cm2 current, the voltages of the fabricated blue LEDs and DUV LEDs were 2.95 and 6.01V, respectively.