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A commercial purity aluminum was heavily deformed up to an equivalent strain of 4 at various temperatures and strain rates by torsion deformation to produce specimens with various ultrafine grained (UFG) microstructures. The microstructures were characterized by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The microstructural observation revealed that the torsion deformed specimens had various mean grain sizes ranging from 0.38 to 8.6 µm. The grain size and dislocation density in the microstructures depended on the deformation conditions organized by ZenerHollomon parameter. The mechanical properties of the torsion deformed specimens were investigated by tensile test at room temperature. It was found that the ultrafine grained specimens showed high strength which reached a value almost three times higher than that of the starting material. The strength of the UFG aluminum was higher than the level expected from the Hall Petch relationship for conventionally coarse grained aluminum. The strengthening mechanisms in the UFG aluminum were discussed in terms of substructures introduced during torsion deformation.