Abstract: The fracture cracks of 9Cr18 stainless steel internal gears were analyzed through chemical composition analysis, mechanical property testing, metallographic structure analysis, macroscopic inspection and SEM morphology observation, with the nature of the cracks finally determined and the possible cracking causes analyzed.The results show that the internal gears are treated by quenching + low-temperature tempering to a hardness of（60±2）HRC, with large structural stress during the martensitic transformation process, and the material structure is in the state of tempered martensite + carbides at this time. The presence of carbides will significantly reduce the material’s crack propagation resistance, greatly increase the brittleness of the material and also significantly enhance the stress sensitivity of the parts. Coupled with the stress concentration parts inherent in the parts, the above comprehensive factors lead to the cracking of the internal gears. The improvement measures are to add small round corners at the sharp corners in the structure, and the surface at the transition or sharp corners should not have burrs or protrusions to reduce the risk of stress concentration, and magnetic particle testing is used to detect surface cracks of the product after heat treatment, so as to ensure that the processed parts are free of cracks through this measure.