Magnesium alloys often exhibit cracking and segmentation after equal-channel angular pressing (ECAP) at room temperature. With torsion shear deformation and a hydrostatic stress, high-pressure torsion (HPT) has an advantage over ECAP in the processing of hard-to-deform materials like magnesium alloys at room temperature. In this report, HPT was used on extruded AZ31 Mg alloy at temperatures of 296, 373 and 473 K for 1 and 5 turns. After HPT processing, the hcp crystal c-axis rotated from the disc (r,θ) plane towards the torsion axis. The angle between the c-axis and the torsion axis (Φ) has a relationship with the HPT processing temperature. It was found that the c-axis was 10º from the torsion axis at 296 and 373 K but 5º from the torsion axis at 473 K. The activity of the basal <a> slip and the twinning exert significant contributions to the deformation. Microstructural features such as the grain size and grain size distributions were examined and correlated with the mechanical properties through the microhardness values.