Mo5Si3-particle-reinforced Si3N4-matrix composites were fabricated by sintering molybdenum-solution-infiltrated porous Si3N4. Fine Mo5Si3 particles, with an average diameter of similar to0.13-0.17 mum, grew in situ from the reaction between MoO3 and Si3N4 in the grain boundary of the Si3N4. The Mo5Si3 particles resided in the grain-boundary glassy phase and reinforced the grain boundaries. The four-point flexural strength and fracture toughness of a 2.8 wt% Mo5Si3-Si3N4 composite were 1060 MPa and 7.7 MPa(.)m(1/2), respectively. This was higher than those for normally sintered Si3N4 by similar to17% and similar to18%, respectively. The fracture toughness of the Mo5Si3-Si3N4 composite increased as the content of Mo5Si3 particles increased, but the flexural strength decreased. Improvement in fracture toughness was attributed to a thermal expansion mismatch among the Mo5Si3, the Si3N4, and the grain-boundary amorphous phases in the Mo5Si3-Si3N4 composites. Another reason for the improved fracture toughness was the pullout of elongated Si2N2O grains that formed as a result of oxygen gas released from the reaction between the molybdenum-solution-obtained MoO3 and the Si3N4. The infiltration method for incorporating desired elements or compounds into a ceramic matrix holds promise as a process for fabricating submicrometer- or nanometer-sized composites with high strength and high fracture toughness.