Friction stir butt welding (FSW) between A5052-O aluminum alloy plates with a thickness of 2 mm was performed.The rotation speeds of the welding tool were 2000 and 3000 r/min,respectively.The traverse speed was ranged from 100 mm/min to 900 mm/min.The defect-free welds with the very smooth surface morphology were successfully obtained,except for at the welding condition of 3000 r/min and 100 mm/min.The onion ring structure was observed in the friction-stir-welded zone (SZ) at the condition of 2000 r/min and 100 mm/min.For all the welding conditions,the grain size of the SZ was smaller than that of the base metal,and was decreased with the decrease of the tool rotation speed and with the increase of the tool traverse speed.The stir zone exhibited higher average hardness than the base metal.The decrease of the tool rotation speed and the increase of the tool traverse speed resulted in the increase in the average hardness of the SZ.The tensile strength of the FSWed plates was similar to that of the base metal,except for at the welding condition of 3000 r/min and 100 mm/min.The total elongation of the FSWed plates was lower than that of the base metal. Friction stir butt welding (FSW) between A5052-O aluminum alloy plates with a thickness of 2 mm was performed. The rotation speeds of the welding tool were 2000 and 3000 r / min, respectively. The traverse speed was ranged from 100 mm / min to 900 mm / min. The defect-free welds with the very smooth surface condition were successfully obtained except for at the welding condition of 3000 r / min and 100 mm / min. The onion ring structure was observed in the friction- welded zone (SZ) at the condition of 2000 r / min and 100 mm / min. For all the welding conditions, the grain size of the SZ was smaller than that of the base metal, and was decreased with the decrease of the tool rotation speed and with the increase of the tool traverse speed. the stir zone exhibits higher average hardness than the base metal. the decrease of the tool rotation speed and the increase of the tool traverse speed resulted in the increase in the average hardness of the SZ. The tensile strength of the FSWed plates was similar to t hat of the base metal, except for at the welding condition of 3000 r / min and 100 mm / min. the total elongation of the FSWed plates was lower than that of the base metal.