Strengthening mechanism related to {1012} twinning plays an important role in improvingmechanical properties of Mg alloy. In this work, a hot-rolled AZ31 Mg alloy sheets were subjected to dynamicplastic deformation (DPD) along the rolling direction and tests were interrupted at strains of 1%, 3% and 5% withthe aim of introducing {1012} twins, and then were annealed at 200℃ for 3 h to eliminate the dislocations. The tensile deformation behavior and the microstructural evolution of these pre-deformedsamples containing {1012} twin lamellar structure with the aim of analyzing effects of untwinning and{1012} twin lamellar structure on the mechanical properties of materials were investigated. When subsequent tensile deformation is carried out along the DPD direction, untwinning causes a significant increase in the maximum flowstress. And the tensile yield stress and the maximum flow stress increases significantly with the volume fractionof twins. The texture hardening which is caused by the texture change attributable to untwinning plays an importantrole in improving mechanical properties of materials. When tension is carried out along the initial transversedirection, slip dominated plastic deformation and {1012} twinning activity is restrained. The tensile yield stress increases slightly with pre-strain, suggesting that the hardening contribution of initial grain refinement by {1012} twinlamellae is not very significant during deformation.