Nanosized tungsten carbide powder was prepared by a thermal plasma process using tungsten hexachloride (WCl6) as the precursor. The reduction and carburization of the vaporized precursor by methane-hydrogen mixtures produced nanosized WC1-x powder, which sometimes contained WC and/or W2C phase. The effects of the molar ratio of reactant gases, plasma torch power, the flow rate of plasma gas, and the addition of secondary plasma gas (H-2) on the product composition and grain size were investigated. The tungsten carbide powder produced by the plasma process showed particle sizes less than 20 nm. The produced powder was heated in hydrogen to fully carburize the WC1-x, and W2C phases to the WC phase as well as to remove excess carbon in the product. Finally, WC powder with particle size less than 100 nm was obtained.