Photoluminescence of GaInP epilayers under hydrostatic pressure is investigated. The &Ggr; valley of disordered GaInP shifts sublinearly upwards with respect to the top of the valence band with increasing pressure and this sublinearity is caused by the nonlinear dependence of lattice constant on the hydrostatic pressure. The &Ggr; valleys of ordered GaInP epilayers rise slower than that of the disordered one. Considering the interactions between the &Ggr; valley and foldedLandXvalleys, the pressure dependence of the band gap of ordered GaInP is calculated and fitted. The results demonstrate that not only ordering along ⟨111⟩ directions but also sometimes simultaneous ordering along ⟨111⟩ and ⟨100⟩ directions can occur in ordered GaInP. ©1996 American Institute of Physics.