Relationship between microstructure and fracture characteristic of reaction-sintered Si3N4-TiN composite has been studied by optical microscope X-ray diffractometer scanning electron microscope and transmission electron microscope. Reaction-bonded Si3N4-TiN composite containing many residual pores was constructed with $alpha$-Si3N4 $eta$-Si3N4 and TiN crystallines as well as some amounts of residual Si. By the post-sintering the composite was changed to rod-like Si3N4 grains an dense body contained $eta$-Si3N4 and TiN phases. No resction compounds wee formed at the interfaces between Si3N4 and TiN grains. The main fracture mode was intergranular type. and the fracture toughness was found to be 10.4MPa.m1/2due to the crack deflection crack bridging and microcracking mechanisms