Cobalt nanoparticles were synthesized by the hydrogen reduction of cobalt chloride in the gas phase. Concentration of CoCl₂, Reaction temperature, and residence time were chosen as key experimental variables for the control of the particle size. Cobalt nanoparticles ranged from 50 to 90 nm in average particle diameter were produced by through all the experiments. As the CoCl₂ concentration in the flame increased, the average particle diameter increased. Average Particle diameter increased with the increase of reaction temperature. Smaller particles were produced with decrease of the residence time of reactant in the reaction zone.