To well preserve the fine structure of MWCNTs in the fabrication of ceramic matrix composites, an ultra-high-pressure sintering method is employed here. The effects of the MWCNTs on the microstructure and properties of the composites are investigated. The B4C-MWCNTs composites are prepared under the pressure of 4.5GPa at a low sintering temperature of 1300 degrees C for 10min. The results indicate that the densification method is beneficial to protect MWCNTs from destruction and create a good interfacial combination between the MWCNTs and the matrix. The indentation crack length of the B4C-MWCNTs composites decreases and the electrical conductivity increases continuously with the increase of MWCNTs additive amount, reaching the optimum values at 5vol% MWCNTs addition. The microstructural observations indicate that the reinforcement mechanism of the composites is mainly the crack branching, crack deflection, bridging, and pullout of MWCNTs. This finding provides a promising approach for producing more robust ceramics with CNTs reinforcing.