Rotating tool spot joining in low rotating speed range is investigated in this study. Torque of the motor increased with the decrease of the rotating speed of the motor. Joint strength increases significantly with the decrease of the rotating speed. Shoulder plug fracture was observed as a characteristic fracture mode under the long joining time and low rotating speed joining conditions and it obtained the large plastic deformation. The rotational motion of the joining interface was similar with the Rankin vortex. The relationship between the joining interface deformation by the external force and the plastic deformability by the friction heat on this joining process was considered from the experimental results.