We have employed the photoluminescence (PL), surface photovoltage spectroscopy (SPS) and Hall effect measurements to study the effects of (NH4)2Sx treatment on the optical and electrical properties of n-type GaN (n-GaN) in this study. (NH4)2Sx treatment of n-GaN led to the decrease of the surface recombination velocity and the increase of the band-edge emission intensity, due to the accumulation of majority carriers and the repulsion of minority carriers near the (NH4)2Sx-treated n-GaN surface, the removal of the native oxide existed on the n-GaN, and sulfur passivation.