Abstract: The change of mechanical properties and thermoelectric power with the extension of ageing time on the type 17-4PH precipitation-hardened martensitic stainless steel at 400℃ was studied. An empirical formula for the relationship between thermoelectric power and impact toughness was put forward and verified by the main steam isolation valve stem that has been in service for 13 years in a nuclear power plant. Results indicated that with the extension of aging time, the impact toughness of the material decreases, and the yield strength, tensile strength and hardness increase, and the shrinkage of the section and elongation of the crack decrease. The change of the thermoelectric power of the material was exponential correlation with the impact toughness, and the yield strength, tensile strength, hardness and thermoelectric power of the material show a good linear relationship. Good agreement was observed between impact toughness evaluated by thermoelectric power measurement and laboratory test value.