The specimens of Al2O3/Si3N4 compound ceramics which contain 3%, 6%, 9%, 12%, 15%, and 18% nano-Al2O3 additions respectively were obtained by pressureless sintering at 1650 in the nitrogen atmosphere. The bending strength and fracture toughness (KIC) were detected. The microstructure and phase composition of the specimens were analyzed by the means of SEM and X-ray. The results show that Al2O3/Si3N4 compound ceramics can be made only using pressureless sintering. With 9-12% nano-Al2O3addition, the specimens have the top mechanical properties. The bending strength reach 710.86MPa, KIC reach 8.61MPa•M1/2. These excellent properties come from many interwoven nitride distributed uniformly in the Al2O3/Si3N4 compound ceramics matrix, which composed of big and firmly plate-like β- Si3N4, hexagonal Sialon and sheet Si2N2O.