Thermodynamically metastable tungsten carbide (W2C) nanoparticles have been synthesized by mechanochemical alloying (MCA) process. Mg was used as reductive agent and graphite as carbon source. The reduction reaction of WO3 and Mg gave nanometer W powders. Subsequent diffusion reaction of fresh W with carbon produced metastable W2C and a small amount of WC nanoparticles in a W rich environment. WC was then transferred to W2C during the prolonged MCA process. The mechanism of such a chemical transformation has been discussed from a point of view of thermodynamics. The subsequent self-propagation diffusion reaction of fresh W with C source controlled the rate of the whole reaction process due to its slow reaction speed. The final product of the MCA process mainly depends on the composition ratio of W to C in the raw materials. W2C nanoparticles in single phase have been finally fabricated by MCA process at a W rich environment. The structure of the final product has been determined by x-ray diffraction, while the morphology and microstructure of the as-milled W2C nanoparticles have been demonstrated by transmission electron microscopy images.