Nanocrystalline cubic substoichiometric WC1-z powders were synthesized by a mechanochemical method at room temperature, a process of highly reactive energetic ball milling of WO3 + 3Mg mixtures containing excessive graphite as a source of carbon for W phase. The excessive graphite plays a very important role in the formation of cubic phase, otherwise the h.c.p. phase is formed. The entropy evaluation shows that the reduction reaction is a self-propagating process. After millimg for 90 hours and excluding MgO from the product by using HCl solution, the cubic substoichiometric WC1-z powders containing a little excessive graphite with the grain size in the range of 4 to 20 nm were obtained.