The effects of friction stir processing (FSP) on microstructures, tensile properties and work hardening behavior of reduced activation ferritic/martensitic steel (RAFM) under different rotation speeds were investigated. The microstructures of FSP samples were examined by scanning electron microscopy (SEM), Electron back scatter diffraction (EBSD) and transmission electron microscope (TEM). Tensile test results show that the FSP samples not only have high strength, but also have good ductility simultaneously. The FSP samples of 150 rpm exhibit the best ductility with an elongation of about 1.5 times that of the base material (BM). The FSP samples of 200 rpm present a good match between strength and ductility, and the ultimate tensile strength (UTS) and total elongation are increased by 77.2% and 34.2%, respectively. FSP samples reveal lower hardening capacity and lower work hardening exponent than BM. As the rotation speed increases, the hardening capacity and the work hardening exponent of the FSP samples gradually decrease. Both BM and FSP samples can be divided into stage III and IV in the Kocks-Mecking type plot of work hardening rate. The FSP samples present the lower absolute slope value at stage III relative to BM. Besides, the FSP samples of 150 rpm and 250 rpm reveal the smallest and largest work hardening rate at stage IV, respectively.