there are some parameters in friction stir welding (FSW) technique such as tool design, tool rotation speed and tool travel which can be controlled in a precise manner thus controlling the energy input into the system. In this study the effects of these parameters were investigated on microstructure and tensile strength of 5052 aluminum alloy. Roll sheets of this alloy were welded by FSW method at different rotation speeds (400, 800, 1600 and 2500 rpm), welding speeds (50 and 100 mm/min) and tools shoulder diameters (14 and 20 mm). The microstructure results showed that the stir zone (SZ) and thermo-mechanically affected zone (TMAZ) had dynamically recrystallized and recovered respectively. Also the tensile strength of samples welded at tool rotation speeds of 400 and 800 rpm, travel speed 50 mm/min and tools shoulder diameter of 20 mm is similar to that of base metal. The tool rotation speeds of 400 rpm have a good welding ability with higher travel speed and lower tools shoulder diameter.