Reaction‐bonded B 4 C ( RBBC ) was fabricated through molten Si infiltrating porous B 4 C preforms. A preform with a 75% relative green density was obtained by mixing two different sized B 4 C powders. Carbon black added to the preform slightly reduces average pore size, but increases porosity. RBBC ceramics showed a dense and homogeneous microstructure. Vickers hardness was about 15 GP a for RBBC fabricated from a single type of B 4 C powder and could reach 22–23 GP a for the carbon‐added samples after infiltration. Morphological evolution and the influence of the microstructure on the Vickers hardness were investigated and discussed.