Abstract: For the application of robotic FSW, the influence of shoulder shape on axial force during welding was researched. FSW of 6061-T6 aluminum alloy was successfully performed by tools with different shoulder shape of flat and concave. The axial force during welding was measured, recorded, and analyzed. The distribution and variation of axial force were analyzed comprehensively with the aid of an established stress model about the shoulder. The results were shown that weld morphology of the flat shoulder was better than that of the concave shoulder. The average axial force curve fluctuated zigzag in the stable welding stage, while the amplitude of zigzag with the flat shoulder was smaller than the concave shoulder. The lowest axial force was measured in rotating speed of 1 500 r/min and the welding speed of 95 mm/min, when the flat shoulder was used, the lowest axial force was 3 828 N, and the concave shoulder was 4 018.5 N. It was attributed of the lower heat input produced welding with concave shoulder and greater resistance to plastic flow of material. In addition, according to the force analysis, when the concave shoulder was used for welding, a certain axial force was added by the movement in the forward direction, which resulted in a larger axial force.