Abstract: In this paper, the microstructure evolution and impact properties of friction stir welds of 9Cr-1.5W-0.15Ta heat resistant steel were studied. The results showed that due to the double effects of the mechanical stirring of the stir tool and the welding thermal cycle, grain breaking, fully austenitized dynamic recrystallization, dissolution of M₂₃C₆ phase at the grain boundaries and formation of M₃C are materialized in the welds. Higher cooling rate after welding restrains the growth of grains, and promotes martensite transformation. The impact test was conducted in the temperature range of −100 ~ 20 ℃. With the increase of impact test temperature, the impact absorbing energy of base metal and FSW weld metal is monotonously increased, and the impact fracture mode changes from brittle fracture to ductile fracture. Due to the formation of lath martensite and the precipitation of "acicular" M₃C carbide in FSW weld, the hardness of FSW weld increases significantly. At the same temperature, the impact toughness of FSW weld decreases. And, the ductile-brittle transition temperature of FSW weld increases from −50 ℃ of the base metal to −40.2 ℃.