Thin films of aluminium silicon oxynitride have been deposited on conducting (100) silicon wafers by filtered arc deposition (FAD) under nitrogen and/or oxygen gas flow. The influence of the N-2/O-2 flow ratio on the crystal structure, optical and mechanical properties has been investigated. The results of X-ray diffraction showed that the film structure comprised of an AlN crystallite with amorphous Si3N4 and SiOx. The optical properties over the range of 350-800 nm were measured using spectroscopic ellipsometry and found to be strongly dependent on N-2/O-2 flow ratio. The refractive index values of the films were measured to be in the range of 2.2-1.64 at a wavelength of 670 nm for oxygen flow range of 0-100%. The hardness of the films was found to be strongly dependent on the oxygen content in the film. The hardness range of the films was between 10 and 22 GPa and for the stress between 0.3 and 1.2 GPa.