Abstract: The flange bolts of low pressure steam inlet pipe fractured during disassembly in a nuclear power plant. The fracture causes of the bolts were analyzed by means of chemical composition analysis, hardness test, metallographic examination, fracture analysis and stress analysis. The results show that the hardness mismatch between the bolts and the nuts caused the flange bolts of the low pressure steam inlet pipe to bite. In the process of disassembly, under the action of excessive torsional shear force, the bolts had ductile fracture.