Nanostructures and microhardness of a commercial purity Al, three binary Al-Mg alloys and a commercial AA5182 alloy subjected to high pressure torsion (HPT) at room temperature were comparatively investigated using high-resolution transmission electron microscopy, X-ray diffraction (XRD) and high-resolution XRD line profile analysis. The hardness values of HPT samples are twice to three times larger than that of the undeformed counterparts. Grain sizes measured by XRD are in the range 10-200 nm with typical average values ranging from 46 to 120 nm. The hardness values and the dislocation densities increased, whereas, the average grain size decreased significantly with increasing Mg contents. Typical dislocation densities are in the range 1.7 × 10¹⁴ m^-2- 2.3 x 10¹⁵ m^-2. However, local densities in grain boundary and triple junction areas might be as high as 10¹⁷ m^-2. The strengthening mechanisms contributing to high hardness may primarily be attributed to the cooperative interactions of high dislocation densities, grain boundaries and planar interfaces.