Abstract: The butt welding was carried out on the thickness of 6mm 7075 aluminum alloy plate using FSW. The dual diameter samples were designed and tested by hydraulic servo testing machine. The deformation and failure process of FSW joints were observed by Olympus microscopes and scanning electron microscopic. The results show that double necking appears on the FSW joint during the process of loading. The necking firstly appears in the retreating side and then in the retreating side with the progress of loading. Microcracks initiate in the boundary of grain in the advancing side and retreating side by the coalescence of microvoids. With the increasing of strain, the amount of microvoids increases. When the strain is large enough, the microcracks appear due to coalescence of microvoids. The microcracks propagate towards the weld nugget zone along the 45° angle to the loading direction and result in fracture of FSW joints. The location of fracture lie in the weld nugget zone and the fracture mode of the joints is shear fracture mixed dimple.