In the present investigation, the design of experiments (DOE) approach was used to optimize both the tool rotation and welding speeds for the friction stir welding (FSW) of A356 Al-Si alloy. Experiments were designed using 32 factorial design technique. Plates of A356 alloy were welded at tool rotation speeds of 300, 500, and 800 rpm, and welding speeds of 12, 16, and 20 mm/min. Models were developed to calculate the size of both α-Al grains and Si particulates in the stirred zone (SZ) of welded A356 plates, as functions of both tool rotation and welding speeds and also their combined effect. A comparison was made between measured and calculated data. It has been found that the calculated results were in good agreement with measured data.