Abstract: 6082-T6 aluminum alloy welded joint with root defect by friction stir welding was analyzed through DIC observation combined with SEM observation to reveal effect of root defects of welded joint on the tensile properties of aluminum alloy friction stir welded joint. The results showed that with the increase of welding speed,the root of the joint was prone to defects and the severity of defects would increase. Due to the root defect,the tensile properties of friction stir welded joint of 6082-T6 aluminum alloy was significantly reduced,and the bearing capacity and strength of the welded nugget zone were reduced. The tensile fracture morphology was step-like with brittle fracture morphology,and there were shallow and small dimples on the step surface.