Abstract: Bobbin tool friction stir welding（BT-FSW） was successfully utilized in the welding of aluminum alloy with hollow or sealed structures, while formation and mechanical properties of the joint were decreased by the manufacturing accuracy or assembling gap of the workpieces. Focusing on the problem, effect of the assembling gap（0 ～ 1. 0 mm） on formation and mechanical properties of 4. 0 mm thick AA6082-T6 BT-FSWed joint were investigated. The results indicated that with the increase of assembling gap, materials in the upper and bottom shoulders were not capable of filling the welds and voids defects were easy to occur in the bottom-shoulder affected zone under the effect of sucking-extruding, which decreased mechanical properties of joints. Formation and mechanical properties of the joint were superior under the assembling gap of 0 ～ 0. 3 mm, tool rotational velocity of 700 r/min and welding speed of 300 mm/min.