A commercial AZ31 Mg alloy was subjected to ECAP (equal-channel angular pressing) and the tensile properties were examined at room temperature. It is shown that the microstructure before ECAP significantly affects the performance of ECAP without breaking samples. When the initial structure before ECAP is homogeneous with equiaxed grains, subsequent ECAP is feasible at lower temperatures. The grain refinement is achieved more effectively as the temperature for ECAP is lowered and the number of ECAP pass is increased. The tensile ductility was reduced after 1 pass of ECAP at 473 K or 498 K. However, the ductility was improved with an increasing number of ECAP pass or by annealing at 473 K or 523 K after ECAP. The results demonstrate that a homogeneous distribution of fine equiaxed grains is important for ductility improvement in the AZ31 Mg alloy.