High-strength Cu/Al2O3 composites usually exhibit obviously deteriorated electrical conductivity when compared with their low-strength counterparts. In this work, a chemical and mechanical alloying-based strategy was adopted to fabricate an ultrafine composite powder with low-content reinforcement and construct a combined structure of Cu ultrafine powders covered with in-situ Al2O3 nanoparticles. After consolidation at a relatively lower sintering temperature of 550 ℃, high-volume-fraction ultrafine grains were introduced into the Cu/Al2O3 composite, and many in-situ Al2O3 nanoparticles with an average size of 11.7±7.5 nm were dispersed homogeneously in the Cu grain interior. As a result, the composite showed an excellent combination of high tensile strength (654±1 MPa) and high electrical conductivity (84.5±0.1% IACS), which was ascribed to the synergistic strengthening effect of ultrafine grains, dislocations and in-situ Al2O3 nanoparticles. This approach using ultrafine composite powder with low-content reinforcement as a precursor followed by low-temperature and high-pressure sintering may have the potential to be applied to large-scale industrial production of high-performance oxide dispersion strengthened alloys.