Abstract: The effect of Zn on microstructure and mechanical properties of biodegradable Mg-1.38Si alloy was investigated. The results show that Zn has preferable modification effect on Mg₂Si phase in as-cast Mg-1.38Si alloy. The morphology of primary Mg₂Si phase changes from polyhedral to finely particle-like. With the increase of Zn content, the size of eutectic Mg₂Si phase gradually decreases. The mechanical properties of as-cast Mg-1.38Si alloy were improved by Zn addition. The tensile strength, elongation, compressive fracture strength and plastic deformation of the alloy gradually increase with the increase of Zn content.