Compressive creep behavior of a hot-rolled Mg-Y binary alloy and Mg-Y-Zn ternary alloys containing small addition of zinc was investigated at 650 K. Creep strength was significantly improved by addition of zinc to Mg-Y binary alloys. The minimum creep rate of Mg-Y-Zn ternary alloys containing 0.02 mol% zinc was ten times lower than that of the Mg-Y binary alloy. In the Mg-Y-Zn ternary alloys, many "planar defects" were observed on a (0001) plane in magnesium matrix during heat treatment, while no such a defect was observed in the Mg-Y binary alloy. These planar defects acted as obstacles to the non-basal slip of the dislocations. Furthermore, the a-dislocations were extended widely on the basal planes. The partial dislocations interacted strongly with each other, so that the dislocation mobility decreases.