As-cast Mg-3Al-3Sn (AT33) alloy exhibits strong strain hardening ability, which is illustrated by the very high H-c and n values. Decrease in generalized stacking fault (GSF) energies in AT33 alloys by Al and Sn additions facilitates the production of dislocations and activation of deformation twins, causing the accumulation of dislocations at twin boundaries, resulting in a high dislocation density in deformed alloys during tension deformation, which thereby mainly accounts for the strong strain hardening ability. (C) 2011 Elsevier B.V. All rights reserved.