Abstract: The microstructure and mechanical properties of the weld of 304 austenitic stainless steel used in PWR(pressurized water reactor) nuclear powder plants reactor internals after thermal aging at 325 ℃, 365 ℃ and 400 ℃ for different time were studied and compared with those before thermal aging. The results show that, after thermal aging, the microstructure in the weld and the main alloy elements contents in ferrite and austenite all had no obvious changes, the micro-hardness of the weld increased obviously but that of the austenite had no obvious changes, and the impact energy sharply decreased and the tensile properties changed less. The higher the thermal aging temperature, the greater the changes of the mechanical properties of the weld.