Abstract: GaAs epilayers were grown on exacdy（100）-oriented Si and 4°misoriented（100）Si substrates by low-pressure metalorganic chemical vapor deposition（LP-MOCVD）.Two-step method was used to the heteroepitaxial growth and the initial amorphous GaAs buffer layers were optimized,respectively. X-ray diffraction（XRD）and transmission electron microscopy（TEM）were employed to characterize the quality of GaAs epilayers.The full width at half maximum（FWHM）value of XRD （400）ωscan is 340arcsec for 1.8μm GaAs epilayer on 4°misoriented（100）Si,compared to 494arcsec for 1.8μm GaAs epilayers on exactly（100）-oriented Si.The images of TEM shows the dramatical reduction of dislocations in GaAs epilayers on 4°misoriented（100）Si substrates.