The amorphous GaN（a-GaN） films are deposited by direct current planar magnetron sputtering in different compositions of argon-nitrogen mixtures. X-ray diffraction patterns and the Raman spectra indicate that the films have amorphous structures. Spectroscopic ellipsometry shows that the refractive index and the film thickness increase with increasing argon content. The UV-Vis spectra indicate that the band gap of the film deposited without argon is 3.90eV, which is much larger than that of crystalline GaN （c-GaN）, owing to the structural disorder. When the films are deposited at higher argon content, the band gap becomes much smaller （2.80—3.30eV）, probably because of more excessive Ga in the films. The band tails extending to lower energies are modeled. Two parameters representing the energy broadening of the electronic transitions and the width or slope of the exponential tail are 0.257—0.338eV and 1.44—1.89?eV, respectively, which are higher than that of c-GaN films, indicating that a-GaN films have wider absorption tails than the c-GaN films. Photoluminescence peak at 360 nm observed at room temperature comes from band_to_band emission.