Friction stir spot welding (FSSW) is a recent solid state welding technology. This welding technique is energy efficient, environment friendly, and versatile. FSSW is a novel derivative of Friction-Stir Welding (FSW) which is proving to be a better alternative to Resistance Spot Welding (RSW). In this work FSSW of aluminium alloy 6061-T6 and mild steel sheets of thickness 1.5 mm was evaluated using circular pin tool. Tool rotation speed, plunge depth and dwell time were varied to determine the effect of individual process parameter on lap shear separation load. Process parameters were optimized by using Taguchi technique. The optimum values for processing parameters were obtained as 2800 rpm rotational speed, 0.9 mm plunge depth and 8 sec dwell time. Maximum lap shear load of about 2250N was obtained. Joint interface is very clean no inter metallic compound is observed at the Al/Fe interface.