Abstract: The 40Cr steel gear shaft was fractured incipiently during use. The fracture cause of the gear shaft was analyzed by macrographic inspection, chemical composition analysis and metallographic inspection. The results show that during the machining process, the flanging and curling scrap was generated on the surface of gear shaft due to too large cutting depth or blunt cutting tool. The surface grain shed down and cutting extrusion produced the microcrack. Meanwhile, too much nonmetallic inclusions existed in the raw material. As the result of it, the strength degraded and the brittlement increased significantly, and cracks were formed due to quenching stress concentration. The cracks continued to expand under the service load, and eventually led to the fracture failure of the gear shaft.