A novel passivation film on GaAs surface has been grown by microwave sulfur glow discharge technique.Auger electron spectroscopy and Rutherford back scattering measurements show that the film is mainly composed of sulfur and gallium,and atomic ratio of gallium to sulfur is about 1∶1.From X-ray diffraction spectroscopy measurememt,GaS is identified to be hexagonal polycrystalline material.The refraction index and dielectric constant of GaS have also been determined.