Compressive creep behavior of a hot-rolled Mg-Y binary alloy and Mg-Y-Zn ternary alloys containing small addition of zinc (0.02 - 0.04 mol%) has been investigated at 550 K to 650 K. Creep strength is substantially improved by small addition of zinc to Mg-Y binary alloys. The minimum creep rate of Mg-0.9Y-0.02Zn decreases to 1/10 of Mg-1.1Y binary alloy. Many "planar defects" on (0001) matrix planes were observed within grains in Mg-Y-Zn ternary alloy, while typical solid solution microstructure was observed in Mg-Y binary alloy. The formation of planar defects suppresses the non-basal slip of the alpha-dislocations because they act as obstacles to the non-basal slip of the alpha-dislocations.