The objective of this study was to understand the work-hardening behavior of a rolled AZ31 magnesium alloy sheet upon reverse loading. Firstly we carried out an in-plane compression-tension test of a rolled AZ31 magnesium alloy sheet with various compressive strains. The sigmoidal curve was exhibited during tension regardless of the amount of compressive strain, but a shape of the curve was clearly different depending on the compressive strain. To understand the mechanism of this difference, a crystal plasticity finite-element simulation was carried out. The simulation result showed that the above difference in the shape was owing to the difference in the activities of slip and twinning systems during tension depending on the compressive strain.