The mechanical properties and degradation performance in Hank’s solution of the directionally solidified (DSed) Mg-0.8Ca (wt.%) alloys were investigated and compared with those of the as-cast alloy. The microstructure was studied by OM, SEM, TEM, and EBSD. It is found that the columnar grains in DSed alloys have a consistent growth orientation of The mechanical strength and ductility of Mg-0.8Ca alloy are significantly improved after directional solidification. Besides, the corrosion resistance of Mg-0.8Ca alloy in Hank’s solution is dramatically enhanced after directional solidification. The Mg-0.8Ca alloys in Hank’s solution mainly undergo microgalvanic corrosion, and the corrosion products consist of Mg(OH)₂, (Ca,Mg)₃(PO₄)₂, and hydroxyapatite (HA). The superior properties of the DSed Mg-Ca alloy are beneficial from the columnar microstructure and the redistribution of the eutectics. The DSed Mg-0.8Ca alloy with columnar dendritic structure is expected to be used as biodegradable materials.