Abstract: The valve stem used in the main steam system of nuclear power plant (NPP) is usually 17-4PH martensitic stainless steel. When it serves in 300 ℃ for a long time, the thermal aging embrittlement of valve stem will be significant, and the performance is that the ductile brittle transition temperature (DBTT) and the hardness increase, and the upper stage energy (USE) decreases. It will seriously affect the safety and economic operation of nuclear power plant. The 17-4PH martensitic stainless steel valve stem after thermal aging was studied by scanning electron microscope (SEM) and electron backscattered diffraction (EBSD). The thermal aging embrittlement behavior of the 17-4PH martensitic stainless steel and the brittle fracture mechanism of the material were further analyzed. The results show that the size of martensite plate bundle and the number of grain boundary increase. The number of microcracks increases on the impact fracture, and the size is similar to that of the martensite plate bundle. Combining with the impact properties, the brittle fracture mechanism of the material was analyzed. The results show that the interaction of the small angle grain boundary and Cu leads to hardening embrittlement, which is the main reason for the thermal aging embrittlement of 17-4PH martensitic stainless steel.