Lightweight aluminum matrix composites were fabricated by the pressure infiltrating molten pure Al or 6061Al into the packed preforms of hollow-structured cenosphere fly ash. The optical microstructure of the composites demonstrated a uniform distribution of the fly ash in the aluminum matrix and seldom infiltration of cenospheres. The interface was clean in the fly ash/pure Al composite. However, interfacial reactions were found in the 6061 matrix composite, and the product was identified as MgAl2O4. The current work provides a better understanding on the interfacial microstructure of the lightweight cenosphere fly ash/Al composites.