A W2C-nanoparticle-reinforced Si3N4-matrix composite was fabricated by sintering porous Si3N4 that had been infiltrated with a tungsten solution. During the sintering procedure, nanometer-sized W2C particles grew in situ from the reaction between the tungsten and carbon sources considered to originate mainly from residual binder. The W2C particles resided in the grain-boundary junctions of the Si3N4, had an average diameter of similar to60 nm, and were polyhedral in shape. Because the residual carbon, which normally would obstruct sintering, reacted with the tungsten to form W2C particles in the composite, the sinterability of the Si3N4 was improved, and a W2C-Si3N4 composite with almost full density was obtained. The flexural strength of the W2C-Si3N4 composite was 1212 MPa, similar to34% higher than that of standard sintered Si3N4.