Marker Fe grains were inserted into 10mm LF2 aluminum alloy in different positions during the friction stir welding course. After butt welding, the samples which were selected in horizontal sections in different depths were analyzed by using EDS(energy dispersive X-ray spectroscopy) of SEM(scanning electron microscope). The 2-D horizontal flow status of friction stir welding for 10mm LF2 aluminum alloy was confirmed by observing the horizontal distributions of Fe grains in welding zone. The results show that the flow is not symmetric about the weld centerline. The lower metal of weld is completely affected by pin. The bulk of metal in the advancing side is transported backward against the welding direction. However, a small amount is moved forward at the edge of the pin. The metal in the retreating side is only transported against the welding direction and some metal flows into the advancing side. The upper metal of weld is mainly affected by shoulder. The bulk of metal in the advancing side moves forward in the welding direction and a little backward; but no metal flows into the retreating side. However, the bulk of metal in the retreating side is transported backward into the advancing side along the rotation direction.