Abstract: The driving and driven gears of N2 sensor of a certain machine have serious wear and some teeth are broken.The material brand of the driving and driven gears is 12Cr2Ni4A steel,and the surface is treated by cyanidation.The properties of gear wear and fracture were studied by means of macroscopic fracture observation,energy spectrum composition analysis,scanning electron microscope observation and metallographic observation.The results show that the failure of driving and driven gears is caused by fatigue fracture and wear caused by abnormal matching.