The aging and creep behavior of die-cast MRI230D magnesium alloy has been investigated and compared with that of a similar alloy, AXJ530, and MRI153M. Samples were aged for 10s to 500h at temperatures from 175 to 350°C. Hardness changes with aging were measured using a micro-Vickers test with 500 gf load. A small increase in hardness, from 68.7±5.5 Hv in the as-cast state to 74.7±3.6 Hv was observed after aging for 100h at 250°C. Microstructure changes were assessed by transmission electron microscopy (TEM). For MRI230D, nanometer-sized precipitates lying parallel to the basal plane were observed in the as-cast and aged conditions. Precipitate morphology, orientation and aging behavior were similar to that observed for AXJ530. Mg 17Al12 precipitates were observed in interdendritic regions in aged MRI153M but were not observed after aging in either MRI230D or AXJ530, and no precipitation of Al2Ca was observed in the primary α-Mg phase in MRI153M.