In this study, GaN metal-semiconductor-metal (MSM) photodetectors (PDs) prepared with and without Ni treatment were fabricated. From I-V measurement, it was found that the fabricated device annealed in O-2 ambient at 600 degrees C, 90 sec exhibited lower dark current by five orders of magnitude than conventional one. Further, it was also found that the UV-to-visible rejection ratio of GaN PDs with Ni treatment is 12146 while that of GaN MSM PDs without Ni treatment is 102. The smaller dark current and higher responsivity observed from the PD prepared with Ni treatment indicated that we could improve the performance of GaN-based MSM PDs by using Ni treatment methodology. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.027204esl] All rights reserved.