Abstract The microstructure and tensile properties of a vacuum-assist high-pressure die casting (HPDC) Mg-3.0Nd-0.3Zn-0.6 Zr alloy and the effect of runner design on the externally solidified crystals (ESCs) were investigated. The microstructure of the alloy mainly comprised ESCs (large primary α-Mg grains) formed in the chamber, small primary α-Mg grains crystalized in the cavity, divorced eutectic Mg12Nd and a small amount of Mg12(NdxZn1-x) or Mg12(NdxZnyZr1-x-y) phases in the region near the shrinkage. The average ESCs grain size decreased as the distance from the center increased remarkably. The employment of ESCs collector contributed to the depression of ESCs grain size, particularly in center region. On the contrary, the grain size of small primary α-Mg has overall slightly downward trend from center to skin region, and the size difference was small. The employment of an ESCs collector effectively decreased the number and size of the ESCs, and accordingly both strength and elongation of the alloy were improved. This specially design ESCs collector improve the runner system and optimize the mold design. The elongation of the alloy approached 10%.