In this paper, AA 6063 alloy was friction stir welded under the submerged condition to obtain the optimum welding condition for maximum hardness. Rotational speed, welding speed and tool pin profiles (cylindrical, threaded and tapered) were taken as process parameters. An L9 orthogonal array with three levels and three factors was designed and executed for conducting trials. The analysis of variance (ANOVA) and signal to noise ratio analyses are employed to investigate the influence of different welding parameters on the hardness and to obtain the optimum parameters. The results indicated that a maximum hardness of 60 Hv can be achieved through underwater FSW at the nugget zone.