Abstract: Four tools with a pin eccentricity of 0.1, 0.2, 0.3 and 0.4 mm were designed to friction stir weld lamination by inserting tracer material, and effect of pin eccentricity on metal flow behavior of the weld was studied. The results showed that, the cross section of the weld consisted of shoulder zone, turbulent flow zone, nugget zone and extruded zone, and the formation of the turbulent flow zone was contributed to combined extruding action of nugget zone and shoulder zone. Moreover, the size of deformed metals on the AS is larger than on the RS. In the same welding conditions, with increasing pin eccentricity, the area and width of nugget zone increased firstly and then decreased.The changing trend of transfer height of insert tracer materials was similar, and the maximum transfer height on the AS was bigger than on the RS. The area of nugget zone and the transfer height of insert tracer material were produced by a tool pin with an eccentricity of 0.2 mm. But the maximum width of nugget zone was obtained by a tool pin of 0.3 mm eccentricity. According to the three-dimensional plastic flow of the weld metal, a wave extruding physical model of the pin was proposed.