Al-Si-N thin films were deposited on glass substrates at varying nitrogen partial pressure through magnetron sputtering. The effect of partial pressure of nitrogen gas in the gaseous mixture (argon and nitrogen) on the optical properties such as absorption coefficient, band gap energy, refractive index, extinction coefficient, dielectric constant of the film was investigated and correlation with structure was carried out. Nitrogen partial pressure has a considerable effect on the film microstructure and consequently on their optical properties. The transformation of the film from opaque to transparent is dependent on the partial pressure of nitrogen. The crystallite size decreases from 27 to 19 nm with the rise in partial pressure of nitrogen from 3 x 10(-1) Pa to 6 x 10(-1) Pa but again increases as the pressure reaches 7.5 x 10(-1) Pa. Refractive index of Al-Si-N coatings shows normal dispersive behaviour. The transmittance, band gap energy, optical conductivity and dielectric constant of the films were also studied as a function of nitrogen partial pressure.