Abstract: The force and radial runout of tool during friction stir welding of 2024 aluminum alloy were measured by using octagonal ring dynamometer and high-speed photography. The experiments confirmed that there was a periodic variation in the force patterns along three directions and the periodicity of variation was equal to the time taken for one complete rotation of tool. The periodic force oscillation was mainly caused by the radial runout of tool and the material flow around the tool. The results imply that with the increase of radial runout of tool, the amplitude of variation of these forces linearly increased. The radial runout of tool had a major influence on the amplitude by studying the relationship between amplitude and radial runout of tool. The amplitude increased with the increase of joining speed, while keeping the radial runout of tool and other process parameters consistant.