Abstract: The governor step motor shaft of a power plant fractured during the startup load adjustment process. The fractured motor shaft was analyzed by macroscopic examination, chemical composition analysis, hardness test, metallographic examination and fracture analysis. And the shear stress of the step motor shaft material was checked. The results show that the failure of the governor step motor shaft was low stress and high cycle rotation/bending fatigue fracture. The main reasons for the fracture failure of the step motor shaft were as follows:on the one hand, the stress concentration phenomenon always happened at the variable diameter position of receding groove, which caused fatigue cracks on the surface of the step motor shaft; on the other hand, the existence of inclusions such as sulphide and carbides would reduce the plasticity, toughness and fatigue strength of the material, and further more, it caused stress superposition, reduced mechanical properties, and accelerated fatigue crack formation and expansion.