Specimens of AZ91-1.5Si-0.4Ca and AZ91 with different section thicknesses were prepared by HPDC(high pressure die-casting). The solidification rate of the HPDC specimens was numerically simulated. The microstructure, tensile properties and the fracture morphologySat the ambient temperature and 180_oC of the specimens were investigated. The results show that: 1) Si and Mg form Mg₂Si phase and Ca mainlySdissolves in the matrix. Besides, Si and Ca have some effect of grain refinement on the Mg matrix. 2) The increase of solidification rate causes that the Mg₂Si phase in AZ91-1.5Si-0.4Ca turn from bulky polygons and Chinese script type to fine polygons and short rod. 3)The mechanical properties of AZ91-1.5Si-0.4Ca and AZ91 at ambient temperature and 180_oC increase with the increase of solidification rate. The influence of section thickness on tensile strength of AZ91-1.5Si-0.4Ca is greater than that of AZ91, but the influence on elongation is less than that of AZ91. Furthermore, the tensile strength of AZ91-1.5Si-0.4Ca at 180_oC is higher than that of AZ91 when the section thickness is less than 2.5mm, and the tensile strength of AZ91-1.5Si-0.4Ca at ambient temperature is higher than that of AZ91 when the section thickness is less than 3mm.