In this study, AA 2024-T351 aluminum alloy plates were butt-welded using friction stir welding process at a constant welding speed of 63 mm/min and tool rotational speed of 400 and 630 rpm. Two welding seams were overlapped via shifting the pin towards advancing side or retreating side. The effect of friction stir welding parameters on mechanical and microstructural properties were investigated. Microstructural investigations and micro-hardness measurements were performed on the transverse cross section of welded joints. The decrease in tensile strength is attributed to the increase in tool rotational speed, leading to an increase in frictional heat generation regardless of overlapping of weld seams. Consistent correlation between tool pin type and other process parameters was found to occur, revealing that the higher tensile strength values were obtained in case pentagonal shaped pin was used. It was revealed that tapered threaded pin profile was resulted in producing defective welded joints; defects, located in the stir zone, were found to be effective on deteriorating the mechanical properties of welded joints.