Abstract: Aluminum alloy 2A12 welded joint was strengthened by ultrasonic impact treatment (UIT).The microstructure of welded joint with and without UIT was studied by optical microscope.Surface micro-hardness distribution across the weld joint and that along the depth direction of the cross section of the weld bead were measured by a micro-hardness tester.Residual stress distribution along the transverse direction of the weld joint surface was investigated by X-ray diffraction techniques. The micro-structural strengthening mechanisms via UIT were investigated.Results show that metallurgical structure of the near surface material of the weld zone was densified due to ultrasonic impact treatment.The depth of plastic deformation manifested by the textured structure extended up to about 300μm from the surface.Defects such as gas cavity and shrinkage porosity in the weld zone were reduced substantially after ultrasonic impact treatment.The hardness values of the weld zone and the surface of the welded joint were improved distinctly.High tensile weld residual stress was eliminated and compressive residual stress was induced by UIT.