Bulk plastic forming characteristics were studied for the magnesium alloy, AZ31 in warm backward extrusion. Effects of process conditions such as extrusion ratio, forming temperature, and punching speed were investigated respectively. Variation of microstructure induced by the warm backward extrusion process was observed. Microstructure of the work piece showed evidences of recrystallization under the experiment conditions. It is estimated that in specific punch speed region fast stroke accelerates recrystllization and reduces the forming load.