The ultrafine titanium carbonitride particles (TiC0.5N0.5) with 100 nm in mean size was successfully synthesized by nitridation treatment at ordinary temperatures, 1373∼1473 K of the nanostructured half-stoichiometric titanium carbide (TiC0.5) particles, which were produced by the magnesium reduction of gaseous TiCl4+1/4C2Cl4. In addition, the nitrogen stability for the produced titanium carbonitride particles at various temperatures and vacuum conditions was investigated experimentally and compared with values calculated by an ideal solution model.