Objective　To provide anatomy basis for the placement of lag screw in the acetabular posterior column via lesser sciatic notch.　Methods　The morphology of lesser sciatic notch was observed on 66 sides of innominatum bone specimens (36males and 30 females). Pelvic CT scanning data from 60 adults (30males and 30 females) were imported to Mimics 14.1 for 3D image reconstruction. On the basis of this result, the morphology of lesser sciatic notch was classified.　Results　According to skeleton classification, type “）” “C” “『” “丿” of each station were 44.4%, 30.6% ,16.7%, 8.3% in male respectively. Type“）”“C” “『”“丿”of each station were 73.3%, 3.3%,6.7%,1% in female respectively. According to three-dimensional reconstruction of Mimics classification, type “）” “C” “『” “丿” of each station were 56.7%, 26.7% , 13.3%, 3.3% in male respectively. Type “）” “C” “『” “丿” of each station were 76.7%, 10.0% , 6.7%,6.7% in female respectively.　Conclusion　If type “C”was encountered, the inferior extremity of lesser sciatic notch will affect placement of the lag screw and into a guide pin should be placed after osteotomy. The guide pin is easy to slip and a “step” should be made at pin- point of lesser sciatic notch if the type is “『”.