Abstract: Instrumented impact test was performed on the Nuclear Power Plant primary pipe material, Z3CN20.09M duplex stainless steel aged at 400ºC for up to 3000 hours to characterize the thermal aging embrittlement effect on the Charpy impact properties. The impact test results show that, with the increment of the aging time for more than 100 hours, the ductile displacement decreases while the dynamic yielding load and the unloading slope presenting the crack propagation rate increase. The crack initiation energy, crack propagation energy and the total Charpy impact energy increase slightly then decrease sharply with the increment of the aging time for thermal aging embrittlement effect. The results also indicate that the reduction of the crack propagation energy is the predominant reason of the thermal aging effect on the impact property of Z3CN20.09M steel. Comparing the experimental results with the semi-empirical predictions using the embrittlement kinetics theory, it is found that the predictions agree well with the experimental results. The SEM results show that fracture surface patterns change from ductile fracture with shallow dimples to cleavage brittle fracture with the increment of the aging time.