In the present study, the reliability of transparent NiO gated AlGaN/GaN heterostructure field-effect transistors have been investigated. Compared with the TiN Schottky gated device, the p-NiO electrode can suppress the gate leakage and shift the threshold voltage positively. Pulse-mode and temperature-dependent current-voltage measurements demonstrate that both kinds of devices present good stability. The frequency-dependent conductance measurement confirms that only fast trap is detected for the TiN gated device while both fast and slow traps for the NiO gated one. The slow trap is ascribed to the hole trapping in the type-II staggered band configuration between NiO and AlGaN/GaN, which is also confirmed from the obvious threshold voltage instability in high frequency loop scan curves.