Direct chill cast Mg-Zn-Zr alloys with different amount of Nd and Y additions have been investigated in this study. The interdendritic phases of as-cast Mg-5Zn-0.6Zr alloy were comprised of a small amount of ZnZr and MgZn. They were changed by the addition of Nd to T phase or T phase coexisting with W phase, and the additions of Nd and Y to W phase, occurring with a refinement of dendritic size, an increase of interdendritic phase amount and an improvement in the thermal stability of interdendritic phases. There was a strong dependence of tensile properties on microstructure. The continuous networks of intergranular phases in Mg-Zn-Nd-Zr alloys significantly deteriorated the ultimate strength and elongation, whereas the less continuity of intergranular phases in the Mg-Zn-Nd-Y-Zr alloys favored the ultimate strength and elongation. © 2006 Elsevier B.V. All rights reserved.