Abstract: The valves in the reactor primary loop are important components that related to the nuclear safety.In this case,it was reported that a crack appeared on the sealing surface of a valve in reactor primary loop.The sealing surface is welded with NOREM,which is an iron based hard facing alloy.The reason of the crack was studied by chemical composition analysis,mechanical analysis,metal graph analysis and SEM analysis.The result showed the composition,mechanical property and microstructure of the build-up welding were conformant to the requirements.The sealing surface cracked along the grain boundary and the location near the base metal suffered greater oxidation,which revealed that the crack features were consistent with the weld hot crack.The crack is the intrinsic property of the weld material,and modification to the weld parameter should be executed to mitigate the tendency of hot crack.