Effects of several different additions on microstructures in in-situ Mg2Si/A1-Si composites were investigated. The results show that rare earth elements have a selective modification effect on eutectic Mg2Si and Si structures. Al-Ti-B modifier should be introduced appropriately more into the composite, so as to effectively refine primary α-Al grains. As (SrCl2+RE) added increases from 0.4wt% to 2.0wt%, morphology of primary Mg2Si in a 12.0Mg2Si/A1-7.6Si composite is changed from polyhedron (or octahedron) to more irregular polyhedron with a size decrease from about 21μm to 6μm, while eutectic Mg2Si is changed from coarse dendrites to dispersed slim fibers. (RE+Al-P) can improve microstructures and tensile strength of a 7.9Mg2Si/A1-7.8Si composite; after modified by 0.8wt% (RE+Al-P), fracture surface of the composite is changed from cleavage to a mixed mode.