. (Al2O3)p/Al composites were in situ synthesized from Al-CeO2 system. The reinforcement phases, morphologies of particles and interfaces between particles and matrix were characterized by XRD, SEM, EDX, TEM and SAED. The in situ generated Al2O3 particles have various irregular shapes and disperse uniformly in matrix. The sizes of Al2O3 particles are normally less than 200nm. The interfaces between particles and matrix are clean. Moreover, there are large numbers of high density dislocations and the generated extensive micro-nano subgrains around Al2O3 particles. These novel characterizations contribute to the significant improvement of composites properties.