Abstract: Ultrafine particles are produced by evaporating of Fe₉₀ B₁₀ in Ar atmosphere and in N₂ at 133 3Pa pressure.Magnetic behaviour of the Fe-B ultrafine particles are very different in the room temperature.The mosphlog is identical and difference of the size is not too large between the particles produced in the different inert gas,but Fe content is very different as so crystallization is.The mechmism of the property are discussed.