The effects of in-situ NH3 plasma passivation on the interface between atomic layer deposited HfO2 and Ga-face n-GaN substrate in metal-oxidesemiconductor (MOS) devices were investigated by varying plasma power and exposure time and compared with GaN MOS device without plasma passivation. ALD HfO2-GaN device with in-situ NH3 plasma treatment shows improved electrical characteristics including negligible frequency dispersion at the near flat-band voltage region, lower hysteresis (similar to 10 mV), suppressed oxide capacitance dispersion in the accumulation (2.2%), lower leakage current density (5.21 x 10(-2) A/cm(2) at 1 V), and low interface state density (D-it) of similar to 6.77 x 10(11) eV(-1) cm(-2) at E-c-E-t = 0.3 eV using an optimized plasma passivation exposure time of 10 min and power of 50 W. These results are attributed that NH3 plasma treatment could eliminate carbon species and detrimental sub-GaOx as well as passivate the surface and bulk defects on GaN caused by Ga-N dissociation.