Abstract: By means of macrographic inspection, chemical composition analysis, mechanical properties testing, metallographic examination and energy spectrum scanning, the fracture reason of guide bearing of 42CrMo steel was analyzed. The results show that the fracture was occurred due to non-metallic inclusions and unreasonable heat treatment mechanism. The expectanted uniform tempered sorbite structure was not obtained. Quenching treatment led to volume expansion and stress concentration on the edge of guide bearing，and then microcrack formed at the position of nonmetallic inclusions, finally with the crack extending the guide bearing fractured.