Abstract Torsion experiments on copper wires with different diameters but the same grain size are performed to figure out the individual contribution of plastic strain gradient to size effect. The samples are prepared by electropolishing the wires with an initial diameter of 50 μm at room temperature for different time periods. A significant strain gradient effect is confirmed in wire torsion. Tensile results on the same wires verify that the tensile strength is not significantly affected by the diameters. The Fleck-Hutchinson strain gradient plasticity theory is employed to analyze the torsional responses, giving a material length scale of about 3 μm.