Abstract: The fracture reason of the butterfly valve clack used in the nuclear power plant was analyzed by means of optical emission spectrometer, optical microscope, Brinell hardness tester and scanning electron microscope (SEM). The results show that the failure mode of the butterfly valve clack was intergranular corrosion fracture. The chemical composition of the valve clack didn′t meet the specified requirement of JIS G 4303-2005 and the solution treatment was inappropriate after casting, which were the main reasons of the valve clack fracture. Continuous carbide was precipitated at the grain boundaries of the valve clack and the grain boundaries were sensitized, resulting in the intergranular corrosion fracture along the austenite grain boundaries after long service in the sea containing corrosive elements of S and Cl.