Abstract: The gear in the gear-box on a line was fractured. The causes of gear fracture were analyzed by macroscopic observation, chemical composition analysis, scanning electron microscopy and energy spectrum analysis, metallographic examination and hardness test. The results show that the gear chemical composition did not meet the requirements of the product design standard GB/T 699-2015, and the microstructure of the fractured gear tooth was gross widmanstatten tissue, and there were a large number of non-metallic inclusions dominated by alumina, resulting in a significant decrease in the toughness and wear resistance of the gear. In the service process, the gear was affected by the bending fatigue stress and abnormal microstructure and inclusions for a long time in the transmission of power, resulting in the fracture of the gear at last.