The new way to obtain the high-quality InAs epilayers on the GaAs substrates is a two-step growth process: InAs grown directly on the GaAs substrates as the prelayers under In-rich conditions, and then growth of the InAs layers on such prelayers under As-rich conditions (J. Crystal Growth 208 (2000) 795). The optimized growth condition for this method from the Raman spectroscopy and the low-temperature photoluminescence is the following: first InAs is grown 20 nm thick under In-rich conditions at 500degreesC with the appropriate V/III ratio of 8, then InAs is continuously grown under As-rich conditions at 500degreesC with the appropriate V/III ratio of 10-23. With the spatial correlation model, the line shape of first order longitudinal optical phonon Raman spectra of the InAs epilayers grown on the GaAs substrates has been analyzed. It is found that the theoretical line shapes and experimental results have a good agreement. (C) 2003 Elsevier Science B.V. All rights reserved.