Defects and deformation structures including 0° and 60° full dislocations, 30° Shockley partials, stacking faults and deformation twins in a nanostructured Al-Mg alloy processed by high-pressure torsion were identified using high-resolution transmission electron microscopy. The twinning mechanism previously predicted by the molecular dynamics simulation, i.e., the homogeneous mechanism involving dynamic overlapping of the stacking faults inside grains, was directly verified. A four-layer twin formed by the dynamic overlapping of four stacking faults was experimentally observed. Deformation twins and stacking faults formed by partial dislocations in ultrafine grains were experimentally confirmed. These results suggest that partial dislocation emissions from grain boundaries could become a deformation mechanism in ultrafine-grained aluminum during severe plastic deformation.