In the present investigation, dissimilar AA5754-O and AA6061-O aluminum alloy plates were joined using friction stir welding (FSW). The influences of the tool pin profile, tool rotational speed and welding speed on the microstructural and mechanical characteristics of the friction stir (FS) welded butt joints were studied. The AA5754-O and AA 6061-O plates were located on the advancing side (AS) and on the retreating side (RS), respectively. The FS welded plates were joined using two different tools having two different tool pin profiles, typically, cylindrical threaded (CT) pin and two flat sided cylindrical (TFSC) pin. The plates were joined using two tool rotational speeds, typically, 400 and 800 rpm and two welding speeds, typically, 50 and 100 mm/min. The results revealed that, regions FS welded using the tool with CT pin exhibit finer α-Al grains than those FS welded using the tool with the TFSC pin. In contrast, regions FS welded using the tool with CT pin exhibited lower hardness, ultimate tensile strength (UTS) and percentage elongation than those welded using TFSC pin. Whatever the pin profile, the joints FS welded using 800 rpm and 50 mm/min showed the highest mechanical properties.