Abstract: 2219-O aluminum alloy was friction stir welded to investigate the effect of friction stir welding on its microstructure and mechanical properties in this paper.The microstructures of different zones in the joint were analyzed by optical microscopy,and the mechanical properties of the joint were evaluated by means of tensile tests.Microstructural analyses indicated that the weld nugget zone(WNZ) is composed of fine isometric grains because of continuously dynamic recrystallization,and more precipitates formed in this zone.In the thermal mechanical affected zone(TMAZ),the grains have experienced incomplete recrystallization due to seriously plastic deformation,and some small grains begin to nucleate.In the heat affected zone(HAZ),all the grains are coarsened.Tensile tests showed that the tensile strength of the joint is the same as that of the base metal,and the fracture occurs in the base metal when the tool rotation speed was 800 rpm and the welding speed was less than 400 mm/min.On the other hand,when the welding speed was greater than 400 mm/min,the mechanical properties of the joint significantly degrade and the joint fractures at the defect location because of the formation of a defect in the WNZ.