Boron carbide (B4C) and silicon carbide (SiC) are materials with high hardness and low density what make them very useful in various applications, such armor plates. Hot press sintering of B4C-SiC composites was studied. The powder mixture was obtained in situ by carbothermal reduction using B2O3, SiO2 and carbon black as precursors. Seeds of commercial B4C and SiC were used to accelerate the synthesis reaction. Compositions were prepared to obtain powders after carbothermal reduction with 10, 30 and 50 wt% of SiC. Carbothermal reaction was conducted in argon atmosphere at temperatures up to 1700 °C. The synthesized powders were analyzed by DRX and SEM. Hot pressing at 1850 °C/30 min (20 MPa) in argon atmosphere was applied. Densities higher than 93% of theoretical density and microhardness of 34 GPa were achieved for hot pressed samples.