AZ31 Mg alloy sheets were processed by pre-stretched (PRS) at the angles of 0o, 30o, 45o, 60o and 90o between the tensile direction and the extrusion direction at room temperature. Twinning was thus introduced and gradually increased. The corrosion performance of the twinned Mg alloy sheets was sensitive to the area fraction of twins. Meanwhile, the sheet processing most area fraction of twins (～52%) exhibited the best corrosion resistance. This was attributed to the subdivision of grains by introducing extension twin boundaries. Twinning brought about more grain boundaries that acted as a corrosion barrier and thus improved the corrosion resistance. The microstructure evolutions and corrosion behavior were examined and compared.