Abstract: A model of forces on pin tool in the plunge stage and the steady welding stage during the friction stir welding(FSW) process is established and verified by comparison with experimental data.It indicates that pin root is the weakest portion of the pin tool in either the plunge stage or the steady welding stage.Pin tool can fail from the root if improper design is adopted, so the model is applied to guide design of tools for FSW of Ti-6Al-4V.The results show that proper tool design is vital to FSW for Ti-6Al-4V.Sound welds could be obtained under appropriate parameters.