Abstract: Microstructures and mechanical properties of the whole specimen in the 12 mm thick plate joint obtained by floating bobbin friction stir welding (FBFSW) were studied.The results showed that refined equiaxed grains could be observed in weld nugget zone (WNZ),and the segregation phases were broken up and mostly dissolved into the matrix.The microstructure in the heat affected zone (HAZ) consists of coarse bar-shape recovery grain.The profile of micro-hardness across the weld exhibits exhibits like"W"and the fluctuation in hardness along the thickness direction of joint was small and that the HAZ has serious softening.The fracture analysis showed that the fracture mode of the joint is ductile fracture.When the rotating speed is 600 r/min and the wielding speed is 300 mm/min,the good weld is achieved.The tensile strength of the joint reaches 231 MPa, which is about 79% of the ultimate strength of the base material.