The composition dependence of energy gaps and refractive indices in the AlxGa1-xAs alloy system in the zincblende structure are reported within the pseudopotential framework under the virtual crystal approximation in which the effect of composition disorder is involved. The calculated energy gaps and refractive indices are in good agreement with experiments for the parent compounds, whereas for the AlxGa1-xAs it appears necessary to include the disorder effect. The variation of the refractive index as a function of the photon energy for different concentrations x shows that this later behaves differently for different Al mole fractions x and that the disorder effect on the refractive index depends strongly on the alloy composition x.