6061-T6 Aluminum Alloy plates were welded by using Friction Stir Welding (FSW) at different rotation and welding speeds. The effects of two factors on Ultimate Tensile Strength (UTS), which include rotation and welding speed at 2o tool tilt angle, were investigated. Ultimate Tensile strength increases with decrease the welding speeds and increases the rotational speeds. The rotation speeds have a higher effect on ultimate tensile strength when compared with the welding speeds. Friction Stir Processing (FSP, Double pass) led to increasing the ultimate tensile strength and elongation at same welding and rotation speeds. FSP lead to improve fatigue life and reduce residual stress.