Abstract: The 0Cr13Ni4Mo stainless steel shaft of a steam feed pump of a 300 MW thermal power unit fractured during service. The investigation for the failure including chemical compositions analysis, mechanical properties test, microstructure observation and fracture pattern examination of the shaft was taken to find out the fracture reasons. The results show that there were many micro cracks vertical to base body in the surface chromium coating of the shaft because of chromizing treatment, which not only reduced the fatigue strength of the shaft, but also provided many cracking sources. Under the action of cycled torsion loading, micro cracks in the chromium coating extended continuously to base body of the shaft until fracture.