The solid state dissimilar joining of mild steel and aluminum 5052-O alloy is successfully achieved by friction stir welding (FSW). The 2 mm thick sheets are butt welded using a convex scrolled tool made of tungsten carbide. With a constant weld speed of 75 mm/min, two different tool at rotation speeds of, 800 and 1000 rpm, were employed to determine the feasibility of the joint formation. Macroscopic observation of the cross section confirmed the formation of a sound FSW joint. However, the formation of an intermetallic in the Stir Zone (SZ) is also observed for the both sets of process parameters. Comparatively, better material mixing is observed when the parameters are set at, 1000 rpm and 75 mm/min respectively. The hardness test revealed the presence of three distinct hardness zones in the SZ for the two parameter sets.