Abstract: The fracture reason of a 42CrMo axis was analyzed by means of macroscopic examination, metallographic examination, the effective depth testing of hardening layer, SEM observation and so on. The results showed that the fracture attribute of axis was brittle fracture, and the early brittle fracture was caused by the microstructure defect which was found in the axis material, such as coarse grain, brittle phase of widmanstatten structure, reticulate ferrite, as well as the troostite existed on grain boundaries and so on.