The 12 mm thick cold-rolled sheets of 5083 aluminum alloy were butt jointed by friction stir welding（FSW）at a rotation rate of 600 r/min. Then compare the effort of microstructure,mechanical properties and fracture pattern with the different welding speeds of 100 mm/min and 300 mm/min. The results show that when welding speed is 100 mm/min,the spacing of onion rings is bigger than the 300 mm/min’s. There is no obvious defect in the welded joint and the mechanical property is close to the parent material,which show the welding quality is high. The fracture position occurs in the HAZ,and the model is ductile fracture of the welds. When welding speed is 300 mm/min,there is the unwelded joint defect at the root of weld joint,which causes a serious decline in the mechanical properties of the joint. The fracture position occurs in the NZ,and the model is a mixture of ductile and brittle fracture of the welds.