In this paper effect of process parameter on mechanical properties of friction stir welded aluminum alloy joints using factorial design is calculated. In experimentation, Two Level of Factorial Design is found to be a more effective tool to investigate the interaction effects of parameters on the required response. Various models have been proposed in this context out of that 95% are at an adequate confidence level. In the analysis of stir welding, tensile strength is found to get an increase with a decrease in rotational speed of tool while decreases with increase in the welding speed. At the low rotational speed of tool high welding speed, there is a small increasing response to Tensile Strength. But at high rpm, and high welding speed, there is a significant increase in Tensile Strength. At a low value of welding speed and high rotational speed, there is a sharp decrease in Tensile Strength. At high welding speed, an increase in rpm, there is a little decrease in Tensile Strength. Prediction of the Tensile Strength at any combination of the two parameters welding speed and rotational speed can be done.With increase shoulder diameter, there is a decrease in Impact Strength. Impact Strength increases with the increase in tool rotational speed.