Aluminum nitride(AlN) thin films have been grown on silicon(100) substrate using a filtered DC arc deposition process. Ellipsometry are used to study the films. According to the deposition characteristics,a fitable model were proposed to fit the refractive index,extinction index and thickness of the AlN thin films. The adhesion of films and substrate was analyzed and different refractive index between films and block was interpreted. The results show that the films′ refractive index and extinxtion index was about 2.015 and zero. The way of thin films adhered to substrate was simple adhesion and the reason that the refractive index of thin films was less than block material was attributed to voids mixed, the ratio of A1/N unequaled 1 and surface oxygen(Al2O3 adhered to the surface).