Abstract: 6061-T6 aluminum alloy thin sheets of 1 mm in thickness were welded by friction stir welding. Process parameters for the welding were investigated, and the mechanical properties of the welded joint such as Vickers hardness, tensile strength and elongation were tested. Metallographic microscope, SEM and TEM were used to analyze the microstructure of the welded joint. The result shows that for 6061-T6 aluminum alloy thin sheets of 1 mm in thickness, the best welding condition is the rotating speed of 1800 r·min^-1 and the welding speed of 1000 mm·min^-1. Under this condition, the Vickers hardness, tensile strength, and elongation of the joint are about 80%, 103%, and 54% as large as those of parent material, respectively. These mechanical properties are in agreement with the microstructure.