A thermal plasma process has been applied to the synthesis of nanosized tungsten carbide powder with ammonium paratungstate (APT) as the precursor. The reduction and carburization of vaporized APT produced nanosized tungsten carbide (WC1-x) powder, which sometimes contained a small amount of W2C phase. The effects of reactant gas composition, plasma torch power, the flow rate of plasma gas, and the addition of secondary plasma gas (H-2) on the product composition and particle size were investigated. The produced tungsten carbide (WC1-x) powder was < 20 nm in particle size. The synthesized powders were also subjected to a hydrogen heat treatment to fully carburize the WC1-x and W2C phases to the WC phase as well as to remove excess carbon. Finally, WC powder of particle size < 100 nm was obtained.