Process for producing ultrafine particles

A process for producing ultrafine particles each coated with a thin film, said method comprising the steps of:

• introducing materials for producing the ultrafine particles into a thermal plasma flame having a center axis, the materials being introduced along the center axis of the thermal plasma flame in a chamber under reduced pressure to form a vapor-phase mixture;
• introducing a mixed gas of a reactive gas and a cooling gas toward an end portion of the thermal plasma flame through an annular slit-like nozzle on said chamber and surrounding the outer periphery of the thermal plasma flame and having a substantially conical shape with an axis that is substantially identical with the center axis of the thermal plasma flame and whose diameter is gradually reduced in a direction to the end portion of the thermal plasma flame in supply amounts sufficient for quenching the vapor-phase mixture to generate the ultrafine particles;
• and
• allowing the resultant ultrafine particles to come into contact with the reactive gas so as to produce the ultrafine particles whose surfaces are coated with the thin film including one or more components derived from decomposition and/or reaction of the reactive gas.