Significant phase refinement and dispersion were achieved in a cast Mg-8 mass%Li alloy through processing by equal-channel angular pressing (ECAP) using a die having an internal channel angle of 110° and a pressing temperature of 473 K. Following extrusion and subsequent ECAP through 2 passes, the alloy exhibited superplasticity over the temperature range from 423 to 473 K. A maximum fracture elongation of >950% was attained at 473 K with an initial strain rate of 1.0×10^-4 s^-1. The measured strain rate sensitivity was of the order of 0.6. The maximum elongation is high in comparison with other Mg alloys tested in tension at the same temperature and strain rate. The activation energy for superplastic flow was essentially equal to the value for grain boundary diffusion of Mg. The use of ECAP gives a significant improvement in strength and ductility at room temperature.