Abstract: In an oil field, the edge of 40CrNiMo steel shear ram of blow-out preventer cracked. In order to find the fracture reasons of the ram edge, fracture morphology observation, microstructure analysis, chemical composition analysis, mechanical properties testing were used to analyze the fracture. The results indicate that, for 40CrNiMo steel ram edge, brittle fracture occurred under the role of greater impact force; The occurrence and propagation of cracks were caused by lack of ductility and inclusions particles at the grain boundaries of 40CrNiMo steel. The main reasons of edge fracture were lower tempering temperature or shorter holding time during the process of quenching and tempering heat treatment.