Abstract The structural, electronic energy band structure and linear optical properties In x Ga 1 −x P alloys are investigated by an ab initio pseudopotential method using density functional theory in the local density approximation (LDA) and a scissors approximation. For these alloys lattice parameters, bulk modulus, band gap energy and density of state (DOS) are calculated. Electronic band structure shows that In x Ga 1 −x P alloys are direct band gap and the optical band gap decrease from 2.10 to 1.41 eV with increase in In concentrations. The linear energy dependent dielectric functions and some optical properties such as energy loss function, absorption and refractive index are calculated. Our results agree well with the available data in the literature.