In the present work, straight cylindrical tool of three different shoulder diameters all made of High speed steel for the friction stir welding (FSW) aluminium alloy 6061 and elongation of the welded joint are tested by universal testing machine. The detailed mathematical representation is simulated by Minitab 15. In this investigation, an effective approach based on Taguchi way has been developed to determine the optimum conditions leading to higher tensile strength. Experiments are conducted by using L9 orthogonal array in Taguchi method. The present work aims at optimizing process parameters to achieve high Elongation. In Elongation the rotational speed play a key role in FSW. The shoulder dia. and feed rate does not contribute the main effect in FSW.