Abstract: Friction stir welding of 7050 aluminum alloy under different rotation rates and tool traverse speeds was conducted and the influence of welding parameters on the grain size of nugget zone was investigated.To analyze the affecting mechanism of welding parameters on the grain size of nugget zone, isothermal compression experiments under different strain rates and deformation temperatures were performed and the influence of deformation parameters on the dynamic recrystallization was analyzed.Results showed that grain size in nugget zone changed slightly with the increase of the rotation rate, while it decreased with the increase of tool traverse speed.Within the range of complete dynamic recrystallization, the recrystallization grain size decreased with increase of the value of lnZ.The welding parameters affected the Zenner-Hollomon parameter and then determined the grain size in nugget zone.