In this article, friction stir welding experiments were performed on AA 5083-H111 aluminum alloy using a tool with a triangular pin to study the effect of tool rotational speed and welding speed on the ultimate tensile strength, percentage of elongation (), hardness and microstructure. The investigations were done with the optical microscopy, stereomicroscopy, scanning electron microscope and energy dispersive spectroscopy. In the visual inspection, it was observed that some of nugget zones have an onion ring like features and a basin-shaped profile. The ultimate tensile strength and percentage of elongation () of the welds decrease with decreasing the welding speed for the 800 r/min. On the other hand, as the welding speed increases, the UTS and decrease for the 1250 rpm/min, the ultimate tensile strength and percentage of elongation () decrease. The main factor, which affects the reduction in both ultimate tensile strength and percentage of elongation (), is defects in the welds. A comparison of ultimate tensile strength and percentage of elongation () as a function of tool rotational speed for the 100 mm/min showed that both ultimate tensile strength and percentage of elongation () decrease with the increase in the tool rotational speed. The weld efficiency varied between 88.3% and 92% in the defect-free welds. The hardness distribution of the zones on the welds revealed that no remarkable differences exist between them. Hence, the variance on the grain sizes of zones is small.