PURPOSE: To obtain a surface-treated ultrafine particle of AlN having low oxygen content and resistant to the increase of oxygen content with time by reacting Al with NH3 gas by high-frequency hot plasma process and heat- treating the obtained ultrafine particle of AlN in an inert gas or in vacuum prior to the contact of the particle with air.

CONSTITUTION: Ultrafine particles of AlN produced by the above reaction are heat-treated in an inert gas or in vacuum at 100-800°C before contacting the particle with air. The treatment is effective in eliminating adsorbed NH3 gas, surface charge and crystal defect existing on the surface of the fine particle and causing the increase of impurity oxygen contained in the ultrafine AlN particle. Accordingly, ultrafine AlN particle having high stability of the surface with time and low impurity oxygen content can be produced by this process.