A high-pressure hot-pressing process was applied to densify a commercial boron carbide-titanium diboride (B4C-TiB2) powder mixture. Nearly fully dense (98.6％) materials were obtained at 1700 ℃ under a pressure of 100 MPa. Compared to the sintering temperature required to achieve similar results when a pressure of only 30 MPa was applied, the sintering temperature was found to decrease by about 200 ℃ under pressure of 100 MPa. Analysis of the thermodynamics and microstructure showed that the plastic deformation of the B4C grains induced by high pressure dominated the densification mechanism when high pressure was applied. Furthermore, higher pressure resulted in remarkably improved mechanical properties of the composites, which could be traced back to the generation of stacking faults in the B4C grains and aggregation of TiB2.