The effects of (Gamma) X-valley mixing in (AlAs)n(GaAs)n (001) superlattices (SLs) on the electronic and optical properties are theoretically investigated versus the SL period (n equals 1.25) and the band offsets. The calculations are based on both the empirical sp3s* tight-binding model and the surface Green-function-matching method. The results show that the top state of the valence band is always confined to the GaAs slabs, whereas the bottom state of the conduction band (CB) shows different behaviors as it is sensitive to band mixing and, therefore, can control the optical properties of the superlattice. For large values of n (n >= 9) the SL is found to have a direct bandgap at (Gamma) -point and, thus, forms of type-I heterostructure consisting of multiple quantum wells. Whereas for the ultrathin-layer SLs (n <EQ 8), the interaction between the wells becomes considerable and band mixing effects are important. The X-valley, attributed to the AlAs slabs, becomes the lowest state in the CB and therefore the SL forms a type-II heterostructure with an indirect bandgap. These valley-mixing effects are shown to be efficient only for small conduction band offsets (CBO <EQ 190 meV). As the SL bandgap energy varies from 2.1 to 1.6 eV when n changes from 1 to 25 respectively, and these former values do, indeed, lie within the energy spectrum of the visible light, we concluded that the studied SLs are suitable for photonic device applications when the layer thicknesses are LGaAs >= LAlAs >= 26 angstroms.