In this studies, Mg(Al)-50 vol.% SiC particle composites were fabricated by pressure infiltration with the two different matrix base materials of Mg and Mg-6Al alloy. Thermal conductivities of the two composites were compared and the effect of addition was studied using X-ray diffraction and scanning electron microscopy (SEM). The analysis of the X-ray diffraction shows that the Al-addition to the Mg causes a contraction of the Mg lattice parameters, which brings a better matching between the matrix and SiC particle. The microstructure and the fracture surface of the composite were characterized by using SEM. The alloying element Al exists mainly in the form of α solid solution, which has a uniform distribution in matrix. From calculation, the interfacial thermal barrier resistance of the Mg-6Al/SiCp composite is about one order of magnitude lower than that of the Mg/SiCp composite.