Abstract: To investigate the torque characteristics in bobbin tool friction stir welding (BTFSW) of 6061 aluminum alloy, the torque in BTFSW was measured, the frequency spectrum and the peak of torque were analyzed, and then the relationship between the torque oscillation and the surface shape of welded joint was also discussed. The research indicates that the torque in BTFSW is periodic. The major frequency of the torque is close to 2 times of the spindle frequency. The frequency difference illustrates that the stacking behavior exists between the tool and the metal flow at different velocity. The experimental results indicated that the peak of torque increases with welding speed being increased and decreases with rotation speed being increased when interface temperature of tool and workpiece was lower than 500℃. Smaller torque oscillation will not affect the surface shape of welded joint, but if the interface temperature was higher than 550℃, the torque will appear abnormal oscillation under adverse condition, such as the too short distance between top and back shoulders, slow travel speed, high rotation speed. This research will be useful to indicate the mechanism of BTFSW, 3-D force characteristics and the relationship between welding parameters and welding quality.