The effect of nitrogen–oxygen ratio on composition, micro–structure and optical properties of AlO_xN_y films deposited by ion beam assisted deposition (IBAD) was investigated by using XPS, XRD and spectroscopic ellipsometry. As the ratio of nitrogen to oxygen decreases, the content of nitrogen in the films decreases, whereas, the content of oxygen in the films increases; at the same time, the amorphous phase in the films transfer to AlON phase. The zero value for extinction coefficient of the films indicates the deposited films are dielectric materials. And the refractive index of films within the whole wavelength range decreases with decreasing the ratio of nitrogen to oxygen.