A dense alumina-silicon carbide (Al2O3-SiC) nanocomposite was synthesized in situ from the reaction of mullite, aluminum, and carbon by reactive hot pressing (RHP). Transmission electron microscopy investigation showed that in situ-formed, nanometer-sized SiC particles were mainly entrapped in the matrix grains, whereas submicrometer-sized particles were located at the grain boundaries or triple points of the Al2O3. In addition, no amorphous phase was observed at the interfaces of the Al2O3 and SiC grains, which indicated strong direct bonding. Fracture-surface analysis by scanning electron microscopy revealed an intrafracture mode. The bending strength of the nanocomposite RHP-treated at 1800degreesC was 795 +/- 160 MPa, and the fracture toughness, measured by the indentation method, was 3.1 MPa.m(1/2).