In this paper, we present a technique to fabricate normally off GaN-based high-electron mobility transistor (HEMT) by sputtering and post-annealing p-NiO x capping layer. The p-NiO x layer is produced by sputtering at room temperature and post-annealing at 500 °C for 30 min in pure O 2 environment to achieve high hole concentration. The V th shifts from −3 V in the conventional transistor to 0.33 V, and on/off current ratio became 10 7 . The forward and reverse gate breakdown increase from 3.5 V and −78 V to 10 V and −198 V, respectively. The reverse gate leakage current is 10 −9 A/mm, and the off-state drain-leakage current is 10 −8 A/mm. The V th hysteresis is extremely small at about 33 mV. We also investigate the mechanism that increases hole concentration of p-NiO x after annealing in oxygen environment resulted from the change of Ni 2+ to Ni 3+ and the surge of (111)-orientation.