The Mg-10Y master alloys made by different processes but with the same composition have obviously different modifying efficiencies and structure heredity effects. The results indicate that there exist differences in the microstructures and grain sizes of Mg-10Y master alloys in the states of conventional casting, as-extruded, heat-treated and rapid solidification. The Mg-10Y master alloys are added to AZ31 magnesium alloy as modifier and an obvious refinement is observed. The final magnesium alloys are all composed of α-Mg, Mg17Al12, Al2Y and Al3Y phases. Similar structures exist in both the Mg-10Y master alloys and the final magnesium alloys. After addition of as-extruded Mg-10Y master alloy, the mechanical properties of AZ31 alloy reach the maximum at room temperature, whose ultimate tensile strength is up to 199.3 MPa and the elongation is up to 9.2%. The corresponding differences of the microstructures and the mechanical properties between Mg-10Y master alloys and AZ31 magnesium alloy are related to the structure heredity of Mg-10Y master alloys.