The effects of the yttrium and zinc additions on microstructure and mechanical properties of Mg-Y-Zn alloys were investigated. It was found that the addition of yttrium increases the eutectic temperature of Mg-Y-Zn alloys greatly. The addition of yttrium can also greatly increase the dynamic recrystallization (DRX) temperature of Mg-Y-Zn alloys. The volume fraction of DRX grains in Mg97Y2Zn1 alloy is larger than that in Mg96Y3Zn1 alloy but smaller than that in Mg95.5Y3Zn1.5 alloy due to the effects of yttrium and zinc addition. The long period stacking (LPS) structures of 18R and 14H were observed in Mg-Y-Zn alloys. The increase in the yttrium content results in increase in strength and decrease in elongation in Mg-Y-Zn alloys. The increase in both yttrium and zinc contents results in increase in both strength and elongation in Mg-Y-Zn alloys. The high strengths of the alloys were thought due to the strengthening by the grain refinement, solid solution strengthening, strain strengthening, high density of plane faults of the LPS structures, and distribution of fine Mg24Y5 phase.