Abstract: AES (Auger Electron Speotroscopy) was used to study the composition depth profile of In_1-xGa_xAs_yP_1-y-InP single heteroju notion fabricated by two-phase supercooling LPE (Liquid Phase epitaxy) method. The variation in initial growth tem perature of the quaternary layer covers the range of 631℃~641℃ and the cooling rate varied from 0.7℃/min to 1.2℃/min. It has been realized that the atom concentration in percent of each composition of the thin quaternary layer, whereof the thickness is less than 0.5μm, does not vary with the depth Z. The atom concentration in percent of phosphorus C_P (Z) in the compositional transition layerof the heterojunction interface versus the depth Z can be formulated experimentally as a hyperbolic tangent function.