It is well known that magnesium alloys have difficulties in room temperature formability because of their HCP structure. As a basic approach to enhance a cold formability, a new combination process including an extrusion followed by a cold equal channel angular pressing (ECAP) was attempted. ECAP die has an inner die corner angle of 135 degree, the fillet angle of 45 degree and thickness of 5mm. A finite element analysis with a three-dimensional thermo-coupled elasto-plastic model was also carried out to understand the change of stress and strain during ECAP. Experiments showed that the AZ31 alloy, which is extruded at a ratio of 20 and is heat-treated at 350°C, was successful in a cold ECAP. From the simulated results, it was found that the effective strain gradually decreased from the inner die side (0.533) to the outer die side. This was confirmed by the analytical analysis via von Mises criterion. Furthermore, it also matched well with the experiments, which showed a uniform shear deformation band. It was also interesting to note that compressive yield strength was drastically increased, which is caused by the occurrence of numerous twins spread across the materials during a cold ECAP.