Abstract: Using macro analysis,the observation of microstructure and fracture morphology and hardness testing,the failure analysis of breaking of 40Cr steel automobile semi-axle was analyzed.The results show that the crack was mainly caused of the decarburized layer on the surface of axial bench connected half shaft flange and rod,the crack initiation was formed under the high torsion fatigue shearing stress.The 40Cr steel contained more non-metallic inclusions,and its mechanical properties could not reach the request as the improper heat treatment process.The fatigue crack formed on the surface extended rapidly under the stress,which resulted in the fatigue fracture.