Friction stir welding (FSW) for 5mm thick Al-Li alloy rolled sheet material has been completed with a cone-shape screw thread pin. The metallurgy experiment demonstrates that the dynamic recrystallization occurs in the weld nugget zone (WNZ), fine equiaxed grains form, and a large number of segregation phases appears at grain boundaries. The microstructure in the heataffected zone (HAZ) consists of coarse bar-shape recovery grain, and that of the thermo-mechanically affected zone (TMAZ) exhibits bent band-like character, but the deformation degree at the advancing side is bigger than that at the retreating side. Microstructures in TMAZ are also recovered, and the amount of recovery grain at retreating side is more than that at advancing side. Tensile test shows that the joint strength and elongation are 345 MPa at welding rate v=40 mm/min, and 9.6% at v=60mm/min respectively. Hardness measurement shows that the FSW joint is soften during welding, and the softened zone at advancing side is wider than that at retreating side. Fractographs confirm that the fracture of the joint is mixed mode of ductile and brittle fracture.