In1-xGaxP epitaxial layers were grown on (100) GaAs substrates by liquid‐phase epitaxy using supercooling technique. The lattice mismatch normal to the wafer surface between In1-xGaxP layer and GaAs substrate varies linearly with the supercooled temperature of the growth solution. The composition‐pulling phenomenon was not observed in this study. The growth rate, the intensity, and the full width at half maximum of the photoluminescent spectrum are also dependent on the supercooling temperature. It is shown that the narrowest full widths at half maximum of photoluminescent peak are 10.6 and 35 meV at 14 and 300 K, respectively, when ΔT is 6 °C, and the strongest intensity is occurred at ΔT=12–18 °C. Carrier concentrations of undoped epitaxial layers are in the range of 1016 cm-3 measured by capacitance‐voltage method at 300 K and Hall method at 77 and 300 K. The optimum growth condition was then determined.