Si3N4-Si2N2O composites were fabricated with amorphous nano-sized silicon nitride powders by the liquid phase sintering (LPS). The Si2N2O phase was generated by an in-situ reaction 2Si3N4(s)+1.5O2(g)=3Si2N2O(s)+N2(g). The content of Si2N2O phase up to 60% in the volume was obtained at a sintering temperature of 1 650 ℃ and reduced when the sintering temperature increased or decreased, indicating the reaction is reversible. The mass loss, relative density and average grain size increased with increasing the sintering temperature. The average grain size was less than 500 nm when the sintering temperature was below 1 700 ℃. The sintering procedure contains a complex crystallization and a phase transition: amorphous silicon nitride→equiaxial α-Si3N4→equiaxial β-Si3N4→rod-like Si2N2O→needle-like β-Si3N4. Small round-shaped β-Si3N4 particles were entrapped in the Si2N2O grains and a high density of staking faults was situated in the middle of Si2N2O grains at a sintering temperature of 1 650 ℃. The toughness increased from 3.5 MPa·m1/2 at 1 600 ℃ to 7.2 MPa·m1/2 at 1 800 ℃. The hardness was as high as 21.5 GPa (Vickers) at 1 600 ℃.