Abstract: Several gear teeth of a secondary driven carburized gear for a feed water pump coupler in a thermal power plant were broken.The fracture cause of gear teeth was analyzed by macro and micro morphology observation, chemical composition measuring, metallographic inspection and mechanical property testing.The results show that the surface hardness of the carburized layer of the gear tooth flank was insufficient, and the wear resistance and fatigue resistance were poor.The surface metal of the gear tooth flank peeled off to form spalling pits under long-term repeated contact stresses.Meanwhile flake defects existed in the microstructure of the gear.The comprehensive mechanical properties of the material were inadequate.During the high-speed rotation of the gear, the tooth flank was subjected to contact stresses, and cracks were initiated at stress concentration areas around the spalling pits and expanded rapidly, resulting in the brittle fracture of the gear teeth.