Abstract: The electric gate valve stem with material of 38CrMoAlA steel and on the main water supply pipe of a power plant fractured during service. Means such as chemical composition analysis, mechanical property test, metallographic test, fracture analysis and energy spectrum analysis were used to analyze the fracture causes of the valve stem. The results show that the fracture of the valve stem was mainly due to improper heat treatment of the valve stem material which resulted in high hardness and seriously low impact toughness of the material, thus brittle fracture of the valve stem occured at the groove to withdrawal the cutting tool as stress concentration location during the opening process.