The Mg alloy rods with a gradient bimodal microstructure were fabricated by forward-reverse torsion at 350 °C. The effects of the bimodal microstructure on tensile properties and deformation mechanism of the alloy were investigated in detail. The formation of gradient bimodal microstructure was attributed to the incomplete dynamic recrystallization and the characteristic of torsion deformation. The bimodal structure had less influence on the yield strength but largely increased the peak stress and tension ductility. The large heterogeneous deformation-induced strengthening via gradient bimodal microstructure was responsible for the enhanced strain hardening capacity and tensile ductility. Finally, the related deformation mechanisms were discussed.