Three kinds of magnesium alloy (AZ31) with different initial microstructure were subjected to tensile deformation. The microstructure before and after tensile deformation were examined. It was found that the deformation mechanisms of the three kinds of magnesium alloy were considerably different. Mg alloy with the coarse grains (about 50mm) along with little dislocation inside deformed by twining and dislocation mechanisms, accompanied with an enormous change of crystal orientation as well as a great increase of hardness. Mg alloy with the fine grains/twins (about 3mm) together with high density of dislocations inside deformed by dislocation mechanisms, accompanied with little change of crystal orientation and hardness. The magnesium alloy with the initial microstructure of fine grains (about 3mm) together with little dislocations inside deformed by dislocation and grain boundary sliding mechanisms, accompanied with little change of crystal orientation, but a large increase of hardness.