Abstract: B₄C based composites toughened by TiB₂ were fabricated by in situ reaction sintering with the original microcrystalline powders B₄C，TiO₂ and glucose. The influences of sintering temperature and additive on the sintering behavior and mechanical properties were investigated. TiB₂/B₄C composites with relative density of 97.7% were obtained by in situ reaction hot-pressing at 1950℃. The composites exhibit improved fracture toughness of 5.3 MPa·m^1/2. (TiB₂，Al₂O₃)/B₄C and (TiB₂，SiC)/B₄C composites with almost full densification were fabricated by using additives of Al₂O₃ and Si powders and sintering at 1950℃ and 1900℃; the fracture toughness of composites reaches 7.09 and 6.35 MPa·m^1/2 respectively. The analysis of microstructure shows that the main toughening mechanism is the crack deflection due to the existence of residual stress.