Nano-sized WC powders were synthesized by vapor phase reaction using the precusor of tungsten ethoxide under helium and hydrogen atmosphere. The phases of the powder were varied with reaction Bone and gas flow rate. The powder size was about 30nm in diameter, and the tungsten carbide powder was coated by carbon layer. The synthesis of nano-sized WC powders was promoted as the hydrogen gas flow rate became higher. Inversely, tungsten oxide was formed by increasing the flow rate of helium gas. The synthesized powders were analyzed by XRD, FE-SEM, carbon analyzer etc.