Abstract: The parkerizing high strength blot fractured during assembly. The analysis on macro and micro,chemical composition,metallographic,EDAS and mechanical property were carried out on the fractured blot. The results showed that the crack surface of blot was oxidized and phosphate similar with bolt surface existed on the crack surface,and there was no decarbonization layer on crack,which showed that the crack existed before parkerizing,so the crack was quenching crack. The bolt fractured was caused by quenching crack.