The liquid‐phase epitaxy (LPE) growth of InGaPAs grown on (100) GaAs substrate has been studied under various growth conditions over the whole solid composition range. It has been found that the degree of supercooling less than 5 °C is necessary to successfully grow InGaPAs LPE layers in the composition region influenced by immiscibility, and that the extraordinary broadening of their photoluminescence spectra is attributed to a large number of defects and dislocations included in them. The role of the elastic energy induced from a substrate is discussed on a basis of the experimental results of the InGaPAs LPE growth on GaAs. The degree of immiscibility is also discussed using the regular solution approximation.