Abstract: An automobile gearbox input axis fractured when straightening after carburizing and quenching heat treatment, and the fracture reasons were analyzed by means of macro and micro fracture analysis, acid corrosion test, metallographic examination and hardness test. The results show that there was residual shrinkage defect at transversal section of the input axis, which made the effective bearing area of the R position reduce and become the structural dangerous section. At the same time, the brittleness of the surface carburizing layer was larger because of the coarse martensite microstructure, so cracks formed at the stress concentrated R position surface and fracture happened under action of the large straightening stress.