Fusion welding degrades mechanical properties of the base metals such as strength and fracture toughness due to microstructure changes. On the other hand, application of friction stir processing（FSP）has been suggested to improve fatigue strength of several aluminum alloy or steel welds. It is known that FSP is an effective surface modification technique to refine and homogenize microstructure of various metallic materials in a solid state, based on the principle of friction stir welding. The microstructural modification in the welded zone using FSP can be also expected to enhance fracture toughness simultaneously. In this study, to investigate fracture toughness of a stir zone（SZ）produced by FSP at the surface of low-carbon steel welds, Charpy impact tests were carried out. FSP provided equiaxed ferrite grains in the SZ, and further grain refinement and cementite fragment were observed with decreasing FSP heat input. The upper shelf energy obtained in the TIG-welded specimens with FSP was much larger than those of the as-welded specimens. In addition, the SZ produced by low-heat-input FSP still remained ductile at -196℃ possible for significant plastic deformation. Thus, low-heat-input FSP could decrease ductile-brittle transition temperature due to significant grain refinement and cementite fragmentation. The results obtained in this study suggest the encouraging prospect of application of FSP as new post-weld treatment for improvement of fracture toughness as well as fatigue strength of the welded steel joints.