It is studied the deformation process by IN SITU surface morphology observation for Mg alloy AZ31 sheet in an SEM. It is found that the deformation starts phenomenologically from twinning process, making out of the first twinning in both intact crystallite grains and existing twinned bands by rolling process, and the secondary twinning in original twinned bands. As the plastic strain increases the twinned bands turn to transverse direction (TD) of the sheet in all three tensile directions: rolling direction (RD), TD and 45° to rolling direction (45). The turning characteristic of grain boundaries were dealt with as well.