This study aims to design a novel inserted welding tool to friction stir spot weld SS400 low carbon steel sheets of 4mm thickness. In order to enhance the efficiency of frictional heat generation and to enhance the quality of the welding spot, the welding tool based on a cylindrical tungsten carbide and is inserted by a SS400 low carbon cylinder. The welding apparatus composed of a vertical milling machine and a welding platform that can keep the load between tool and workpiece constant. The plunge load is 8kN and there's no inclination angle on the tools. Welding temperature and the tool plunge depth are measured by thermelcouples and a displacement meter. ¡@¡@At the tool rotational speed of spindle of 900rpm and welding for 60 seconds, the temperature rising rate of the tools with 5 mm and 10 mm inserted material are 5.28 times and 6.31 times greater than the one without insert. While they are 1.36 and 1.42 times greater than at 1200rpm.At the tool rotational speed of spindle of 900rpm and welding for 300 seconds, themaximun welding temperature the tools with 5 mm and 10 mm inserted material can reach are 59¢J and 412¢J higher than the one without insert. While they can reach 35.6¢J and 197.6¢J greater than at 1200rpm. According to the tensile test, the shear failure loads of clad steel plates increase 11.3kN and 15.5kN by using tools with 5 mm and 10 mm inserted material at 900rpm for 60 seconds, and increase 7.6kN and 18.3kN by using tools with 5 mm and 10 mm inserted material at 1200rpm.