A friction stir welding method includes providing a joining tool including a friction surface, a first workpiece, and a second workpiece, and arranging the first workpiece and second workpiece in position with a first joining surface abutting a second joining surface. A friction surface of a joining tool resists at least one of the first treating surface and the second treating surface, and the first workpiece and the second workpiece cooperatively define a slot. Then positioning, rotating, and moving the joining tool to rub and stir at least one of the first workpiece and the second workpiece, thus plasticizing at least part of the first workpiece and the second workpiece to join the first workpiece and the second workpiece.