In the present paper, the deformation behavior of AZ31 cast magnesium alloy sheet was investigated both experimentally and numerically. Although the sheet had random crystallographic orientations, an asymmetric deformation was observed between tension and compression; the stress level was higher under tension than under compression, and the stress-strain curve was affected by a loading path as in the case of a rolled magnesium alloy sheet. A crystal plasticity finite-element method was used to examine the mechanism that causes such asymmetry. The simulation results showed that the activities of twinning and nonbasal slip were clearly different between tension and compression, leading to the asymmetric deformation behavior in the cast magnesium alloy sheet.