The effect of torsional oscillation on the surface quality of an extruded sidewall of a workpiece was investigated in backward cup extrusion with an extrusion ratio of 2.0 at room temperature. In this extrusion process, a cylindrical aluminum workpiece lubricated with mineral oil was extruded in the axial direction and simultaneously twisted in the circumferential direction between extrusion and knockout punches. The two punches were controlled with an axial speed of 0.1 mm/s, an alternating amplitude of 5°, and a maximum angular speed of 0.5 rpm. The torsional oscillation tilted the traction direction at the container–workpiece from the radial direction to the circumferential direction with approximately 25% lower contact pressure on the container. As the result, the surface of the extruded sidewall was uniformly smoothed.