Abstract We have performed a first-principle Full Potential Linearized Augmented Plane Waves calculation within the local density approximation (LDA) to the zinc-blende Al x Ga 1− x As 1− y N y to predict its optical properties as a function of N and Al mole fractions. The accurate calculations of electronic properties such as band structures and optical properties like refractive index, reflectivity and absorption coefficient of Al x Ga 1− x As and Al x Ga 1− x As 1− y N y with x≤0.375 and y up to 4% are presented. Al x Ga 1− x As on GaAs have a lattice mismatch less than 0.16% and the lattice constant of Al x Ga 1− x As has a derivation parameter of 0.0113±0.0024. The band gap energies are calculated by LDA and the band anticrossing model using a matrix element of C MN=2.32 and a N level of E N=(1.625+0.069 x) eV. The results show that Al x Ga 1− x As can be very useful as a barrier layer in separate confinement heterostructure lasers and indicate that the best choice of x and y Al x Ga 1− x As 1− y N y could be an alternative to Al x Ga 1− x As when utilized as active layers in quantum well lasers and high-efficiency solar cell structures.