GaAs/AlAs heterojunction has been widely used in practice for various devices. Band offsets at semiconductor heterojunctions are fundamental parameters which govern the transport properties to miroelectronic devices. Recent theoretical and experimental results have focused attention on the thin Si interlayers at GaAs/AlAs heterojunctions in determining heterojunction band discontinuities. Both the presence of Si interlayer and GaAs at different growth temperature probably bring crystal defect into GaAs/Si/AlAs, which lead to bad quality of GaAs/Si/AlAs heterojunctions and influence the technical applications of heterojunctions in practice. GaAs/Si/AlAs heterojunctions prepared by MBE are examined. The influence of 0.5 ML Si interlayer and GaAs at different growth temperature on GaAs/AlAs are investigated by DLTS technique. The results reveal that the Si interlayer has little influence on the crystal, whereas the lower growth temperature for GaAs is, the more deep level defects occur in the crystal