[en] Structural, electronic and optical properties of the zinc-blende Ga_1_−_xAl_xP ternary alloys with their ordered AlP and GaP binary compounds have been investigated, using the full potential linearized augmented plane wave method in conjunction with the density functional theory. The total energies are carried out to calculate the lattice constant, bulk modulus and its pressure derivative of the zinc-blende AlP, GaP binary compounds and their corresponding ternary Ga_1_−_xAl_xP solid solutions for the compositions (x = 0.25, 0.50 and 0.75). The band gap energies and the optical properties of these materials are investigated at normal pressure condition as well as under high pressure levels. The estimated results obtained from this work are justified, discussed and compared with the experimental data and other available theoretical works. (paper)