The inertia welding process was investigated using Response Surface Methodology. The optimum welding condition to yield maximum breaking strength at the weld was attained through a steepest ascent path. A second-order predicting equation for weld strength was established without significant lack of fit to the data. The response surface is represented by a family of ellipsoids. The optimum region covers a relatively wide range of factor levels for welding low-carbon steels. The weld is formed by the subsurface material under severe spiral plastic flow after the surface layer has been disrupted and squeezed out. A successful weld has an average hardness 27 percent higher than that of the base material. The hardness has the highest value at the center and gradually declines toward the edge and away from the interface of the specimen.