AA 3003-H24 aluminum alloy plates were butt-welded in order to investigate the effect of friction stir welding process variables on the structural properties of the welded joints. The welded joints were characterized using macro-structural investigations; and tunnel-type defects with varied sizes were detected to occur. At a welding speed of 50 mm/min, tunnel-type defects with large size were found to occur in joints welded with the tool rotational speed of 500 and 1000 rpm. The tunnel-type defects were detected to occur in welded joints produced with the process parameters of 80 mm/min welding speed and 500 and 800 rpm tool rotational speed. However, cavity-type defects were observed to occur at both welding speeds. Even, defects were found to occur in the welded joints, all welded joints were detected to fracture between the base metal and the heat-affected zone, except for the joints welded under the process parameters of 50 mm/min welding speed, 500 and 1000 rpm tool rotational speed. The highest ultimate tensile strength among all the welded joints was measured to be 128 MPa produced under the process parameters of 50 mm/min welding speed and 800 rpm tool rotational speed. It has been revealed that there is a good correlation between the size of the defects and the tensile properties of welded joints. An examination on the fracture surface of the welded joints revealed that welded joints were fractured in ductile manner except the joint produced under the process parameters of 50 mm/min welding speed and 500 rpm tool rotational speed.