Abstract: Root defects of friction stir welded (FSW) joints of 6082-T6 aluminum alloy at different welding speeds (200, 300, 600 mm·min^-1) were analyzed by optical microscope and scanning electron microscope, and the microscopic morphology characteristics of the root defects were determined. The results show that when the welding speed was 200 mm·min^-1, the faying surfaces at root of the joint had a good solid phase connection and the root defects were mainly S curve defects. When the welding speed was 300 mm·min^-1, the root defects of the joint were mainly weak bond defects in the form of discontinuous microcracks. When the welding speed was 600 mm·min^-1, the root defects of the joint were mainly incomplete penetration defects in the form of continuous microcracks. The weak bond defects in the form of discontinuous microcrack existed in the location from the incomplete penetration defects to the weld nudget.