Bulk samples of pure Mg and Mg-Gd alloys were prepared by high-pressure torsion (HPT). The HPT made samples exhibit ultra fine grained (UFG) structure with grain size around 100 nm. Results of microstructure investigations of the UFG samples obtained by positron lifetime (PL) spectroscopy, transmission electron microscopy (TEM) and X-ray diffraction (XRD) are presented. In particular, lattice defects introduced by HPT were characterized. The data obtained at atomistic level are compared with macroscopic properties given by microhardness measurements.