Abstract: The change of nonlinear ultrasonic and mechanical properties of the main steam isolation valve stem in a nuclear power plant at 400℃ for different aging time was studied. The results show that with the thermal aging time increasing, the impact energy of 17-4PH stainless steel decreases, while the Brinell hardness, tensile strength and yield strength increase. Hardness, tensile properties and nonlinear ultrasonic show a clear correlation. Ultrasonic nonlinear parameter is sensitive to the microstructure changes of 17-4PH stainless steel during the aging process, which can be used for nondestructive evaluation and characterization of 17-4PH stainless steel.