Based on the density function theory, the electronic structure, densities of states, and optical properties of -AlON are calculated by using the local density approximation and plane-wave pseudoptential method. The complex dielectric functions and the optical properties including refractive index, reflection spectra, absorption spectra of -AlON are analyzed in terms of calculated density of state. As shown by the theoretical calculation, the static dielectric function of -AlON is 1(0)=2.60, and the refractive index is n0=1.61, its absorptions in a range from IR to NUV and a UV range up to 20.23eV all approach to zero, showing the optical transparent behaviors . The calculated results are consistent with relevant experimental values, providing theoretical basis and reference for the application of the optical material to windows and domes.