We report the growth of InSb layers directly on GaAs (001) substrates without any buffer layers by molecular beam epitaxy (MBE).Influences of growth temperature and Ⅴ/Ⅲ flux ratios on the crystal quality,the surface morphology and the electrical properties of InSb thin films are investigated.The InSb samples with roomtemperature mobility of 44600cm2/Vs are grown under optimized growth conditions.The effect of defects in InSb epitaxial on the electrical properties is researched,and we infer that the formation of In vacancy (VIn)and Sb anti-site (Sbin) defects is the main reason for concentrations changing with growth temperature and Sb2/In flux ratios.The mobility of the InSb sample as a function of temperature ranging from 90 K to 360K is demonstrated and the dislocation scattering mechanism and phonon scattering mechanism are discussed.