Structural, phonon and thermal properties of $\rm{Al_{x}Ga_{1−x}As}$ alloy for different values of x (x = 0.0, 0.25, 0.5, 0.75 and 1.0) have been investigated by quasiharmonic Debye–Einstein model and Quantum Espresso package. The correction of Vegard’s law for lattice constant has been examined and has a good agreement with other experimental and theoretical works. TO–LO splitting at gamma point, which is related to breaking of cubic symmetry, has been calculated by optical phonon mode calculation. It is found that by increasing the amount of Al in $\rm{Al_{x}Ga_{1−x}As}$ alloy, specific heat at constant pressure and Debye temperature will be increased.