In this study, Frictional Stir Burnishing (FSB) was applied to 0.45% C steel plate using a tool with a flat tip. As the result, an enhanced layer of high hardness (650 HV) was formed on the material surface. The surface roughness was large (Ra 1.2 μm), and tensile residual stress (300 MPa) was induced on the processed surface. The FSB-processed surface was further machined by face milling to reduce the roughness, and burrs on the FSB-processed surface were effectively removed. It was confirmed that the surface roughness (Ra 0.2 μm) was reduced, and compressive residual stress (-400 MPa) was induced on the processed surface without decreasing the high hardness (650 HV) within the enhanced layer.