This paper investigates the microstructure, mechanical properties and creep behavior of Mg-Al-Ca alloys with different Ca content. SEM and EDAX analyses show that the dominant second phase in the as-cast Mg-Al-Ca alloys is Al(2)Ca, which distributes at the grain boundaries and disperses in the grain interior as well. Both the elevated tensile strength and the creep resistance of Mg-Al-Ca alloys obviously increased with increasing Ca at high temperature. TEM analyses reveal that finer Al(2)Ca particles with an average size of 0.02 mu m precipitated dynamically during the creep process. Selected area electron diffraction (SAD) patterns show that the dynamic Al(2)Ca precipitates have a coherent interface with matrix as (01 (1) over bar0) Mg // H (220) Al(2)Ca, [2 (1) over bar(1) over bar0] Mg // [ 112] Al(2)Ca. The strengthening mechanism of Mg-Al-Ca alloys at elevated temperature was discussed.