<p>To clarify the mechanism of the forgeability (forging limit without fracturing) improvement of magnesium (Mg) alloy in cold upsetting with cyclic torsion, activities of slip and twinning of Mg alloy during deformation were numerically investigated by crystal plasticity finite element analysis. For simulating upsetting with torsion of a cylinder, compressive and shear deformations were simultaneously applied on a cube with sides of 10 mm. The minimum compressive strain for the asymptotic relative activity of slips was the lowest in compression with oneway shear, while the strain was the highest in compression without shear. The critical strain for sharp increase of the relative activity of basal slip in twinned region was (compression with oneway shear) &lt; (compression without shear) &lt; (compression with cyclic shear). The order of critical strain among the deformation modes agreed to the order of fracture strain among the deformation modes in upsetting experiments. A decrease in the relative activity (slope) means slower rise in the integral. In other words, fracture is less likely to occur.</p>