[en] The formation of fine vanadium nitride powders by a vapor phase reaction of VCl₄-NH₃-H₂-N₂ system was investigated between 700⁰C and 1200⁰C with emphasis on the effects of reaction conditions on the particle size and chemical composition. Oxidation resistance and thermal stability of vanadium nitride powders were also examined. The reaction was performed by two methods concerning the mixing of reactant gases; (A) low temperature mixing (400--640⁰C) and (B) high temperature mixing (700--1100⁰C). The mean particle size and surface area of vanadium nitride powder prepared by method (A) were around 500 A and about 10 m²/g, respectively. Those values of vanadium nitride powder prepared by method (B) were around 100 A and about 70 m²/g, respectively. These vanadium nitrides showed a wide range of nonstoichiometry; atomic ratio N/V=0.96--1.64 (A) and N/V=1.55--1.87 (B). X-ray diffraction patterns showed that all nitride samples had NaCl-structure. The atomic ratio N/V decreased with the increase of reaction temperature or VCl₄ or H₂ concentration, and increased with the elevation of NH₃ concentration. The lattice parameter decreased with the increase of N/V ratio. Density measurements showed that vanadium nitrides obtained had V vacancies. The oxidation of vanadium nitride powders began at about 200⁰C in air. The nitride powders with atomic ratio N/V>1 began to decompose at about 600⁰C under vacuum. On the basis of these results, the formation process of vanadium nitride particles was discussed. (auth.)