A new kind of ultrafine amorphous particles Fe82P11B7have been successfully prepared by the reaction of Fe(III) chloride, sodium hypophosphite and potassium borohydride in aqueous solution. X‐ray diffraction and Mo¨ssbauer spectroscopy confirmed the amorphous structure of the particles and scanning electron microscopy showed that the particles are nearly spherical with the diameters from 150 to 350 nm. The structure relaxation and crystallization of the amorphous particles were also studied by Mo¨ssbauer spectroscopy. It is found that the particles began to crystallize when annealed at 573 K, below which, no crystalline phases were formed, but the amorphous structure significantly varied. The particles crystallized to form &agr;‐Fe and Fe3(P0.6B0.4) completely when annealed at 673 and 773 K.