Abstract: The microstructure,micro-penetration hardness and chemical composition of B7 stud which thread stripped and the same lot number raw material after drawing and tempering were analyzed by means of light microscope,rockwell apparatus,HX-100 micro-hardness tester and Baird DV-6 optical emission spectrometer. The result showed that some microstructure defects in raw material such as banded segregation and inclusion result in the B7 stud longitudinal cracking;the surface decarburization of the raw material made hypo-crack taken place,even the thread stripped.