Binderless tungsten carbide ceramics were produced by reactive spark plasma sintering, using tungsten nitride and carbon black as starting materials. The phase and microstructure evolution during the sintering process was investigated. It was found that the tungsten nitride decomposed into nano tungsten with a particle size of about 25 nm firstly, and then reacted with carbon black to form carbides. Due to the high activity of the in situ formed carbides, samples could be densified at a temperature as low as 1350 degrees C. The bulk carbides possessed an average grain size of 270 nm and Vickers' hardness of 25.4 GPa.