Abstract: To improve the high-temperature mechanical properties of aluminum-based materials with the aim of satisfying application requirements for the structural components in aerospace above 573 K, a novel aluminum matrix composite reinforced with 20vol% volume fraction nano-Al₂O₃ particles (146 nm) was prepared via high energy ball milling followed by vacuum hot pressing, and its microstructures and high-temperature compressive properties were investigated. The results show that, nano-Al₂O₃ particles are uniformly distributed in the ultrafine-grained Al matrix, and the resultant composite is fully densified. The composite exhibits superior high-temperature compressive properties: As the strain rate is fixed as 0.001/s, the high-temperature compressive strength reaches 380 MPa at temperature of 473 K, still maintains a high value of 250 MPa when the temperature increased to 673 K, which is at least onefold higher than that of traditional Al-based materials. By establishing constitutive model based on thermal activation, it is also found that the composite shows high stress exponent which is 30 and high apparent activation energy which is 204.02 kJ/mol. This may be attributed to the addition of high volume fraction nanoparticles into Al matrix which not only anchors Al grain boundaries and enables thermal stable interface between nanoparticles and the Al matrix and thus significantly enhance the thermal stability of the microstructure, but also can impede the dislocation motion effectively as well as Al grain boundary, thereby increasing the threshold stress for hot deformation which ranges from 190.6 MPa to 328.4 MPa as the temperature is in the range of 473 K to 673 K. The hot deformation process of this composite can be properly explained by substructure invariant model.