The effects of welding conditions, tool rotation speed, Rt and plate traveling speed, V on weld joints formation and mechanical properties have been investigated on Friction Stir Welding joints of high strength aluminum alloys of 2024-T6 and 7075-T6 in comparison with 5083-O of 4 mm thick plates. Rt/V, Welding parameter, which is closely related to welding heat input per unit length of the welded joint affected joint qualities. Peak temperature of FSW joints near stir zone during welding increased with increasing Rt/V, but lower than solidus temperature for each alloy. As weld defects, lack of bonding and inner defect occurred at low Rt/V for each alloy, and excess high Rt/V caused surface tearing as defect both for 2024-T6 and 7075-T6. As to a range of optimum welding condition, 5083-O was wider than those of 2024-T6 and 7075-T6, and optimum Rt/V range common to three alloys was 3.3 to 5.0. In case of any aluminum alloy, tensile strength of the welded joints increased with increasing Rt/V, and weld defects caused low tensile strength. The maximum tensile strengths for 2024-T6, 7075-T6 and 5083-O were 373, 444 and 296 MPa in as-welded condition, which were almost 76, 83 and 100% to base metal strength. Post-weld artificial aging increased the tensile strength of 7075-T6 joint to 512 MPa, 90% to base metal strength, but in 2024-T6 decreased it slightly.