Abstract: Fatigue tests were performed on the friction stir welded joints of 2219-T6 aluminum alloy in order to research the relationship between the locations of fatigue fracture of 2219-T6 aluminum alloy friction stir welded joints and local mechanical properties and microstructure. The fracture locations were mostly in the junction of the nugget and the thermal mechanical affected zones and adjacent areas. Big impurities were found in the subsurface of the location of the crack initiation. The obvious micro-void and the maximum amount of low angle grain boundary were found in the nugget. In order to obtain the local mechanical properties of the welded joints,the numerical simulation under fatigue loading for the welded joints was conducted with ABAQUS software. The result shows that principal strain values in thermomechanical and nugget zones are bigger than that in other zones. It easily initiates the fatigue cracks in these zones than in others.