The process of extrusion followed by equal-channel angular pressing (ECAP), designated as EX-ECAP, was applied to a model cast Mg alloy containing 7.5wt%Al and 0.2wt%Zr and a commercial cast AZ61 alloy for grain refinement to the submicrometer range. The thermal stability of the fine-grained structure was examined by conducting static annealing experiments. In order to increase the area fraction of high-angle boundaries, additional ECAP was attempted at a higher temperature. Tensile testing was conducted on the fine-grained alloy at elevated temperatures. It is shown that low-temperature superplasticity or high-strain rate superplasticity may be achieved in the fine-grained sample processed by EX-ECAP.