The microstructure of a pressureless infiltrating 55vol% oxidized SiC preform by Al-8Mg alloy was characterized by transmission electron microscopy (TEM), high resolution TEM (HRTEM), field emission scanning electron microscopy (FE-SEM), and X-ray diffraction. The TEM image of the interface between Al and SiC shows that the surface of SiC is covered by a rough nanocrystal layer of MgAl₂O₄, Al₂O₃, and Si, produced by the interfacial reaction of Al, Mg, and SiO₂ on the surface of SiC. The Al-SiC interface is also examined by HRTEM to be better understood how MgAl₂O₄ and Al₂O₃ are produced. Dendritic Al₂O₃ crystals are embedded in the pores of the composite generated from the mutual bonding of SiO₂ on the surface of SiC. Columnar AlN crystals of about 250 nm in length are bunched vertically on the SiC particle surface.