Mg-5Al-8Zn-xCa( x= 0, 1.75, 2.0, 2.25, 2.5, 2.75 and 3.0 wt%) Mg alloys were prepared by gravity casting. The effects of Ca content on microstructures and mechanical properties of the alloys were investigated by XRD, OM and SEM. The results show that the as-cast Mg-5Al-8Zn-xCa alloy is composed mainly of a-Mg matrix, b-Mg17Al12, Mg32(Al, Zn)49, MgZn2 and Al2Ca phases. With the increase of Ca content from 1.75% to 2.75%, the matrix grain is obvious refinement, the morphological change in b-Mg17Al12 phase occurs from coarse continuous network to fine intermittent network and distributed on the grain boundaries, the lamella-type phase Al2Ca is obvious refinement too, meanwhile the tensile strength of the alloy at room temperature reaches maximum, it is 138 MPa, increased by 27.8%, compared with the alloy with absence of Ca addition. When the Ca content reaches 3.0%, the grain is coarsened once more, meanwhile the tensile strength of the alloy decreased. The tensile fractures of the alloys are characterized by quasi-cleavage at room temperature.