The influence of 0.1-1.0 wt% mischmetal (MM) additives on the microstructures, tensile properties, and fracture behavior of A356 alloys were investigated in detail under the as-cast condition. Experimental results show that, after introducing a small amount of MM, grain coarsening occurred, the eutectic silicon was well modified, and RE-containing intermetallic compounds containing Al, Si, Mg, La, and Ce elements were formed. The size, shape, and distribution of RE-containing compounds are also studied. Tensile tests revealed that MM addition decreases the tensile strength and ductility of the materials. The fracture path goes through the interdendritic regions containing eutectic silicon and RE-containing compounds.