Abstract: In this paper,joints of 2219,7A52 and LF21 aluminum alloy were obtained by using a self-made ultrasonic friction stir welding machine.The microstructures and fractography of these joints were studied.The results show that the thermal mechanically affected zone(TMAZ) of ultrasonic friction stir welding is almost disappearing;The grains of weld nugget zone(WNZ) by ultrasonic friction stir welding are much finer than the ones obtained by conventional friction stir welding;The tensile fracture of base material appears to be mixed fracture with dimple and tear ridges.The tensile fracture of ultrasonic friction stir welding appears to be a dimple pattern.Although the tensile strength of joints obtained by ultrasonic friction stir welding are higher than the ones obtained by conventional friction stir welding,the elongation of the joints fellow on opposite tendency.