Abstract: The influence of the pin shape on the flow behavior of the plasticized metal in the weld during the friction stir welding was investigated.Four kinds of pins were used,that is,(a) cylindrical pin with counter-clockwise screw thread,(b) cylindrical pin with clockwise screw thread,(c) conical pin with counter-clockwise screw thread,(d) cam pin with counter-clockwise screw thread.The results show that there are three types of movement in the plasticized metal in the weld during the friction stir welding,namely,circumferential movement near the surface,screwy movement around the pin and the stiff-plasticized movement beyond the plasticized region.The pin shape is the main factor of affecting the flow pattern of the plasticized metal in the weld.The different screw directions on the pin surface will exert different forces on the plasticized metal.This will drive the metal around the pin to move upward or downward in a screwy manner,thus causing the metal to flow with them.The change of the pin shape will result in the change of the flow feature of the plasticized metal.