To improve the mechanical properties of B4C ceramics, TiB2 was generated in-situ in B4C ceramics by a reaction sintering method with TiO2 as a sintering aid to prepare dense B4C ceramics, and the enhancement mechanism of its mechanical properties was analyzed. The results show that the density and flexural strength of B4C firstly increase and then decrease and the fracture toughness increases with the increase of TiO2 addition. The density of B4C ceramics reaches 99.6% when the TiO2 addition is 5% (in mass fraction) and the sintering temperature is 1 700 ℃. The optimum overall performance of B4C ceramics (i.e., the Vickers hardness of 36.0 GPa, the fracture toughness of 4.38 MPa·m1/2, the bending strength of 405 MPa) can be obtained when the amount of TiO2 added is 15%. The in-situ TiB2 inhibits the growth of B4C grains, eliminates the stress at the crack tip, deflects and bifurcates the crack, and plays a role in grain refinement strengthening and toughening of B4C ceramics.