B4C–TiB2 composite powders were synthesized by a carbide boronizing process, i.e. the solid state reaction between TiC and amorphous boron powders. The particle size of the raw TiC powders was found to greatly affect the phase composition of the composite powders. Nano-TiC powders could be totally consumed and turned into TiB2 phase with heat treatment at 1100 °C for 1 h whereas micro-TiC powders was still present even at 1400 °C for 1 h. The synthesized B4C–TiB2 composite powders have a small particle size and a homogenous phase distribution, indicating high sintering activity.