Al₂O₃-cubic boron nitride (cBN) composites were prepared from Al₂O₃ and cBN powders by spark plasma sintering (SPS) at temperatures of 1200 to 1600°C for 600 s under a pressure of 100 MPa. Densification, phase transformation, microstructure and mechanical properties of the Al₂O₃-cBN composites were investigated. Fully dense Al₂O₃-cBN composites containing 10 to 20 vol% cBN with a relative density of more than 98% were obtained at 1300°C for 600 s without the phase transformation from cBN to hexagonal BN (hBN). The transformation of cBN was more greatly accelerated in the Al₂O₃-cBN composite than in cBN. The cBN phase to Al₂O₃ inhibited the grain growth of Al₂O₃ in the Al₂O₃-cBN composite. Vickers hardness of dense Al₂O₃-cBN composites containing 10 to 20 vol% cBN sintered at 1300°C showed the maximum value of 26 GPa. The Al₂O₃-cBN composite containing 20 vol% cBN sintered at 1300°C had the highest fracture toughness of 4.1 MPa·m^1/2.