GaAs was nitrided in the helicon-wave exited N-2+Ar mixed plasma without and with a short-time pretreatment in inductively coupled O-2+Ar mixed plasma. Nitridation of GaAs was also attempted using the inductively coupled N-2+O-2 mixed plasma. Relatively good capacitance-voltage (C-V) characteristics were obtained as compared with those reported in a previous article, especially for the GaAs samples treated in N-2-Ar plasma with the pretreatment in O-2+Ar plasma and treated in the N-2+O-2 plasma. Positive bias-temperature stress improved, to a certain degree, the C-V characteristics. Photoluminescence at low temperature showed that the interface with good electrical quality was obtained after these N-2-plasma treatments. X-ray photoelectron spectroscopic data indicated that the stable GaN bonds were formed through nearly the whole depth of the grown film, with very small amounts of the suboxides of Ga and As. This indicates that N-2-plasma treatment substantially suppressed oxidation of GaAs.