Abstract: The gas apical axis of a nuclear power plant fractured. The fracture reasons were analyzed by means of macroscopic observation, chemical composition analysis, hardness test, metallographic examination and fracture surface analysis. The results show that there were defects and reticular ferrite in the material. The strength and impact toughness of the material were reduced by the proeutectoid ferrite, and the brittle fracture of the material occurred under the condition of tangential stress.