Si3N4 ceramics were prepared by pressureless sintering at 1 650 ℃ in nitrogen atmosphere using Si3N4 powder as main starting material and adding nano-Al2O3 powder (3%, 6%, 9%, 12%, and 15% in mass, the same hereinafter). The bending strength and fracture toughness (K_IC) of the specimens were detected. The microstructure and phase compositions of the specimens were analyzed. The results show that Si3N4 ceramics can be prepared by pressureless sintering when adding 9%-12% nano-Al2O3 as active reactant, which dissolves in Si3N4, in-situ forming non-oxide SiAlON. The obtained Si3N4 ceramics have the maximum bending strength of 710.86 MPa and K_IC of 8.61 MPa·m^1/2. The excellent properties come from many interwoven structures distributed uniformly in the ceramics matrix, which is composed of big and firm plate-like β-Si3N4, hexagonal SiAlON and sheet Si2N2O.