High-pressure torsion (HPT) was conducted with ring samples of pure Al (99.99%) having different thicknesses. They were cut from an Al plate with 10 mm thickness to dimensions having inner and outer diameters of 23 and 30 mm with thicknesses of 2 mm or 4 mm. HPT was conducted at room temperature under a pressure of 1 GPa at a rotation speed of 1 rpm. It is shown that the strain was introduced more intensely in the mid part on the cross section. This strained region extended with increasing number of revolution and covered throughout the cross section in the 2 mm thickness ring after 30 revolutions. The intense-strained region extended to ~3 mm in the 4 mm thickness ring after 30 revolutions but remains almost the same even after 100 revolutions. The thickness of ~3 mm may be an upper limit to achieve homogeneous introduction of the strained region throughout the cross section.