Abstract: Microstructure and mechanical properties of friction stir welded joints of 9%Cr reduced activation ferritic-martensitic (RAFM) steel were studied in the present paper. The results indicate that there exists significant microstructural difference at different zones in friction stir welded joints. In stir zone (SZ), dynamic recrystallization of austenite leads to grain refinement, martensitic transformation, dissolution of M₂₃C₆ phase and precipitation of M₃C phase. Although microstructure in thermal mechanically affect zone (TMAZ) is similar to SZ, the grain size in TMAZ obviously larger than that in base materials (BM). Both heat affect zone (HAZ) and BM perform tempered microstructural characteristic. The hardness of SZ in joint increases significantly and the distribution is uniform. There is a great variation for hardness in TMAZ. The HAZ is softened and its hardness value is the lowest in the joint. With the increase of the testing temperature, the yield strength decreases monotonically and the ultimate tensile strength first increases and then decreases, while the total elongation decreases first and then increases.