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Grain refinement can significantly enhance the strength of metallic materials, but usually at the sacrifice of ductility. Here we report that refinement of magnesium grains can not only improve its strength, but also its ductility, due to the activation of more slip systems. It is found that pure Mg with coarse grain size (d¯= 125 μm) has a low ductility and uniform elongation (5.3%), due to the limited basal slip systems. In contrast, fine-grained Mg (d¯= 5.5 μm) exhibits enhanced work hardening and ductility as well as uniform elongation (18.3%). Two beam condition TEM analysis revealed that the improved properties were due to the activation of non-basal dislocations, such as< c> and< c+ a> dislocations with reducing the grain sizes. It is also found that the< c+ a> dislocations are unstable and can dissociate into either< c> and< a> dislocations or I1 stacking faults. Contributions of nano stacking faults on strengthening and …