Abstract: The inhomogeneous deformation behavior of friction stir welding (FSW) 2A12-T4 aluminum alloy joint during tension was analyzed on the basis of digital image correlation (DIC) method. The local mechanical properties of the joint were then studied by the iso-stress assumption method, and compared with global mechanical properties obtained by the transverse tensile test. The results show that there was a large strain gradient inside the joint during tension by the DIC method. The local true stress-true strain data of base metal obtained by iso-stress assumption method was consistent with the true stress-true strain curve by the test, which verified the accuracy of the iso-stress assumption method. The calculated local true stress-true strain curve of heat affected zone at the distance of 6.5 mm from the weld center on advancing side was closer to the tensile properties of the joint. The yield strength of the junction of thermal-mechanically affected zone and heat affected zone was the smallest, and this position was the weakest area of the joint. The high strength area of the joint had constrains on the low strength area, which led to the better mechanical properties of the joint than the local mechanical properties of the weakest area.