Reaction sintering of boron carbide represents an attractive densification process. In this work, sintering mechanisms of silicon carbide and boron carbide composites were studied. Mixed boron carbide/graphite mixtures were sintered in a vacuumed graphite furnace between 1380 and 1450oC. The samples were in contact with bulk silicon metal which melts at 1410oC. Reaction sequence of the composition was investigated by X-ray diffraction, SEM and TEM. It was found that a reaction between molten silicon and B4C occurred and the reaction produced silicon carbide and silicon-containing boron carbide. Dense composites can be achieved by pressureless sintering at 1450oC and the final microstructure consists of silicon carbide, boron carbide, silicon-containing boron carbide and residual silicon at grain boundaries.