Abstract: The hot-rolled annealed hypoeutectoid steel plates with a thickness of 3 mm was processed by friction stir processing using K40 cobalt tungsten carbide tool. The microstructures and mechanical properties of the processed zone were investigated. The results show that the microstructure of stir zone and thermo-mechanically affected zone were proeutectoid blocky ferrite, "acicular ferrite" and pearlite. Its transformation characteristic was dynamic recrystallization and phase transformation. However, heat-affected zone was characterized as equiaxed ferrite and lamellar pearlite, which was mainly controlled by recrystallization. Friction stir processing had significant effect on the morphologies of pearlite and the precipitation cementite in above different zones. Microhardness in the processed zone of hypoeutectoid steel sample obviously increased after friction stir processing and its ultimate tensile strength increased by 8.2% compared to that of the base material. As a result, the fracture location of tensile specimens appeared at the base material. The fracture mechanism of samples before and after friction stir processing treatment was dimple ductile fracture. Solid solution strengthening and phase transformation strengthening were the two factors for improving the hardness and the ultimate tensile strength of hypoeuectoid steel sample.