Short mullite fiber reinforced Al-Cu-Si alloy composites were produced by squeeze casting. The interfacial microstructure and microchemistry of the testing composites were studied by means of X-ray diffraction (XRD), transmission electron microscope (TEM) and energy disperse spectrum (EDS) equipped with TEM, respectively. It is shown that chopped mullite fibers in the composites are polycrystalline in structure composed of many fine mullite crystal particles which are orthorhombic in structure with lattice constants of a = 0.749 nm, b = 0.927 nm and c = 0.581 nm based on electron diffraction (ED) analysis and high resolution transmission electron microscope (HRTEM). There are much denser dislocations in the near vicinity of both fiber/matrix interface and silicon/matrix interface. But interfacial reaction products at fiber/matrix interface in (3Al2O32SiO2)/Al-4.0Cu-5.0Si composites have not been observed yet.