The Ultrafine alloy particles of an alloy includes a primary metal and one or more subsidiary metals solid-soluble in said primary metal, a content of the one or more subsidiary metals is in a range of 1 wt % to 25 wt % and the one or more subsidiary metals solid-solved in the primary metal inhibit coalescence or oxidation of the ultrafine alloy particles including the primary metal, or both. The process introduces powder materials including the primary metal and the one or more subsidiary metals for producing the ultrafine alloy particles into a thermal plasma flame under reduced pressure to form a vapor-phase mixture and introduces a cooling gas toward an end portion of the thermal plasma flame in a supply amount sufficient for quenching the vapor-phase mixture so as to generate the ultrafine alloy particles.