Compositional dependence of the ordering probability in GayIn(1−y)P epitaxial layers grown on GaAs substrates is investigated. Experimental results reveal that band‐gap energy reduction due to the ordering is varied with the Ga composition of GayIn(1−y)P and shows a dependence on growth temperature in a wide range of Ga compositions where no serious lattice mismatch occurs. In addition, the change of the ordering of GayIn(1−y)P is confirmed by transmission electron diffraction patterns. For the calculation of the relative ordering probability of the GayIn(1−y)P alloy, a model representing an ordering state with alternative stacking of two tetrahedral micro‐unit cells (three Ga‐P‐one In)/(one Ga‐P‐three In), is proposed. The results of the calculation on the compositional dependence of ordering probability, which are based on internal strain energy states of the unit cells of the GayIn(1−y)P alloy, show good agreement with experimental results. ©1995 American Institute of Physics.