A5052 aluminum alloy thin sheet with 0.6 mm thickness was lap welded by friction welding using a rotating disk. Microstructure and mechanical properties of the welded joint were investigated. Two kinds of welding methods, up-welding and down-welding were used. The method when the disk is rotating in the same direction as the welding is called up-welding while one when rotating in the opposite direction is called down-welding. The joint surface after the up-welding showed relatively flat. A scale-shaped pattern and the unevenness by the deposit were observed on the down-welded joint surface. Both joints showed finer microstructure in the vicinity of weld interface compared with base metal. The hardness of the upper sheet in weld zone of both joints showed slightly higher value in comparison with base metal. Hardness of lower sheet in weld zone of both joints were lower value than that of the base metal. The tensile shear load of the up-welded joints showed higher than that of the down-welded joints. The maximum tensile shear load of welded joint was 2.74 kN at welding speed of V=4mm/s by the up-welding.