The focus of this work is to investigate the effect of seeding on the synthesis of B4C powder obtained by carbothermal reduction. B4C particles were added to act as seeds during reaction. Starting materials were boric acid (H3BO3) and two different carbon sources (graphite and carbon black). Mixtures of C, B2O3 (25 wt% B2O3 excess in relation to stoichiometric composition) and commercial B4C powder (0, 2.5 and 5.0 wt%) were prepared. The effect of 1.5 wt% of NaCl was also investigated. The powder mixtures were uniaxially die-pressed to form cylindrical shaped green bodies, then placed in a graphite crucible and heated at 1750°C/1h with a heating rate of 30°C/min, under argon-flowing atmosphere. Samples with seeds addition exhibited a decrease in particle size distribution. The particle morphology was uniform with a narrow size distribution and a medium particle size around 6 m regardless the initial concentration of seeds.