One of the concerns of this investigation is to control and reduce defects of the holes in the crosssection of the stir zone, which might be created during friction stir processing when low rotational speed and high forward speed are used. These defects were mainly observed at the beginning area of the workpiece, and the main reason is the low temperature of the tool and the sample during the process. Some defects could also be seen at the tail-end due to the tool coming out of the workpiece and the reduction of plastic materials in the leading part. The generated heat by low rotational speed and high forward speed, the high thermal conductivity of Aluminum alloy, and the filling pattern are the main factors causing the poor fluidity of plastic materials around the tool. This, in turn, causes hole defects and leaves some cavities made by the leading tool unfilled. On this basis, rigorous attention was given to the performance of the process.