In this study, the theoretical tool design was proposed based on the material flow during friction stir spot welding. At first, the fracture mode in a cross tensile test and a tensile shear test was investigated in detail in order to propose the ideal profile of the welded joint with a high tensile strength. Based on the material flow model, it became clear that the strong inwards flow produced by the shoulder or the increase of the heat input to soften material around the stir zone leaded to producing the ideal joint with the high tensile strength. Thus four types of tools with different shoulder profiles were investigated. The investigation of the material flow beneath the shoulder efficiently, attained the high joint strength in both a cross tensile and a tensile shear test.