The evolution mechanism of mechanical properties at elevated temperature is particularly crucial to the oxide/oxide composites. In this work, an Al2O3/Al2O3 ceramic matrix composite was prepared by laminating with high solid content slurry. The microstructure, micro- and macro- mechanical properties of the composites before and after thermal aging at 1400 °C were examined. The density and porosity of the original composite were ∼2.60 g/cm3 and ∼27.2 %, respectively. The sintering phenomenon of matrix in the composite after thermal aging was clearly visible, resulting in the matrix modulus increased by 83.5 % and the interfacial shear strength increased by 154.9 % compared with those of the original composite. The fundamental mechanics theory of composites stated that the significant increase of interfacial shear strength was the main reason that the flexural strength retention ratio of the composite decreased to ∼55 % after thermal aging.