The sintering behavior of B4C can be significantly affected by the additions of Si. The B4C based ceramics are prepared via hot pressing. The mixtures that contained initially 15wt% Si sintered at 1830°C for 0.5h, 98.7% relative density reched. The composition and microstructure of the composites are characterized by means of XRD and SEM. Some reactions occurred during the sintering of the two powders. The products are B4C, SiB6, SiC and B sub>12.97 Si.03 C2 when B4C+15wt%Si eutectic composite sintered at 1830°C for 0.5h. The Flexural Strength and Fracture Toughness of composites reached 447.2MP and 4.62 MPa•m sup>1/2 highly be improved respectively.