The microscopic characteristics of the GaAs(100) surface treated with P2S5/NH4OH solution has been investigated by Auger electron spectroscopy (AES) and X-ray photoemission spectroscopy(XPS). AES reveals that only phosphorus and sulfur, but no oxygen, are contain-ed in the interface between passivation film and GaAs substrate. Using XPS, it is found that both Ga2O3 and As2O3 on the GaAs surface are removed by the P2S5/NH4OH treatment, moro-ver, gallium sulfide and arsenic sulfide are formed. The passivation film results in a reduction of surfacer state density and improving both electronic and optical properties of GaAs surface.