Abstract: 17-4 precipitation hardening (PH) stainless steel valve rods are widely used in PWR power plants. When 17-4 PH valve rod has been working under high temperature (around 300-350 ℃) for a long time, it will face the problem of embrittlement due to thermal aging and affect the safety of the nuclear power plant. Small angle neutron scattering (SANS) experiment for real valve rod used in nuclear power plants was conducted. Combined with other methods including Charpy impact test, scanning electron microscope and metallographic microscope, differences between severely aging and slightly aging valve rod samples from nuclear power plants were compared, and changes in nanostructure inside the material during long-term thermal aging were studied. From changes in impact energy and scanning images, brittle of severely aging valve rod can be seen clearly. Polydisperse sphere model and Porod law were used for fitting in SANS analysis. The results show that nano precipitates are basically sphere around 1 nm in diameter. In the process of thermal aging, the size grows and the volume fraction increases about 19%. The result of SANS is clearly correlated to changes in macroscopic mechanical properties.