Abstract: The novel 9Cr-1Mo ferritic heat-resistant steel was selected to conduct friction stir welding experiment adopted tungsten-rhenium alloy tools, and the variation laws of weld formation, microstructure and mechanical properties were discussed. The results indicated that no defect welded joints could be obtained with the rotation speed of 300 and 400 r/min, and welding speed of 50 mm/min; the weld zone was mainly composed of stir zone and thermomechanical affected zone, which had obvious characteristics of hardened martensite microstructure; the high temperature heat affected zone presented mixed microstructures of hardened martensite and tempered martensite, and the low temperature heat affected zone showed over tempered martensite. The grains in the stir zone were refined and the average grain size of the stir zone was about 69.2% of that of the base metal. The weld zone had been significantly hardened, and the maximum hardness in the weld zone was more than 2.0 times of that of the base metal. The ultimate tensile strength of the welded joint reached about 98% of that of the base metal, and the V-shaped notched impact absorbed energy of the stir zone and heat affected zone reached 77.8% and 87.4% of that of the base metal respectively. Those results indicated that the friction stir welded joint still has the better matching between strength and toughness.