The ultrafine titanium carbonitride particles (TiC_0.7N_0.3) below 100nm in mean size were successfully synthesized by Mg-thermal reduction process. The nanostructured sub-stoichiometric titanium carbide (TiC_0.7) particles were produced by the magnesium reduction at 1123K of gaseous TiC_l4+xC_2Cl_4 and the heat treatments in vacuum were performed for five hours to remove residual magnesium and magnesium chloride mixed with TiC_0.7. And final TiC_0.7N_0.3 phase was obtained by nitrification under normal N_2 gas at 1373K for 2 hrs. The purity of produced TiC_0.7N_0.3 particles was above 99.3% and the oxygen contents below 0.2 wt%. We investigated in particular the effects of the temperatures in vacuum treatment on the particle refinement of final product.