Equal channel angular pressing (ECAP) was applied to a wrought magnesium alloy AZ31 for up to 8 passes at temperatures as low as 100 °C. The application of a back pressure was critical in deforming Mg alloys at lower temperatures. Room temperature mechanical properties were obtained by tensile and hardness tests. With increasing ECAP strain, the initial coarse grained structure was transformed into a submicrometer-grained microstructure. In general, hardness increased with decreasing grain size although the changes in tensile strength and ductility were more complicated.