Abstract: Friction stir processing was applied to cool-rolled DC04 IF steel plates with a thickness of 3 mm,and the microstructure and mechanical properties of the processed zone were investigated. A defect-free specimen with a shiny and smooth surface is fabricated at a tool rotation rate of 950 r·min^-1 and a travel speed of 60 mm·min^-1 using additional forced cooling technology after processing.A fine equiaxed grain structure can be obtained in the processed zone,which causes a higher hardness of HV 135.6,1.4 times as large as the base metal at the center of the processed zone,and the highest hardness of HV 312.8 in the surface. The ultimate tensile strength of processed samples with and without the surface fine-grained layer increases by 50.9% and 47.6% compared to that of the base material,respectively. The both fracture surfaces of samples before and after friction stir processing exhibit dimple ductile fracture characterization. Grain refinement is considered to be the main reason for improving the ultimate tensile strength.