Effects of volume fraction of long-period stacking order (LPSO) phase on work hardening of cast Mg-Zn-Y alloys were investigated. Stress-strain curves for Mg-Zn-Y alloys with four volume fractions of LPSO phase were obtained by tension-compression cyclic loading tests with a constant strain amplitude under a constant strain rate. The results showed that the stress amplitude of the alloy with high volume fraction of LPSO phase continuously increased up to 50^<th> cycle. With increase in volume fraction of LPSO phase, the increase in the stress amplitude became significant. The evaluation of the contributions of isotropic hardening and kinematic hardening to cyclic hardening indicated that the amount of kinematic hardening significantly increased during cyclic loading in Mg-Zn-Y with high volume fraction of LPSO phase.