The nanocrystallization process of soft ferromagnetic (Fe0.99Mo0.01)(78) Si9Bi3 ribbons has been studied in detail. Microstructural and ferromagnetic properties are examined by transmission electron microscopy (TEM), X-ray diffraction (XRD), Mossbauer spectroscopy (MS), differential scanning calorimetry (DSC) and magnetization measurements. The Curie and crystallization temperatures are determined to be T-C = 665 K and T-x = 750 K, respectively. The T-x value is in well agreement with DSC measurement results. XRD patterns had shown two metastable phases (Fe23B6Fe3B) which were formed under in situ nanocrystallization process. These metastable phases embedded in the amorphous matrix have a significant effect on magnetic ordering. The ultimate nanocrystalline (NC) phases of alpha-Fe(Mo, Si) and Fe2B at optimum annealing temperature had been observed respectively. It is notable that the magnetization of the amorphous phase decreases more rapidly with increasing temperature than those of NC ferromagnetism, which suggest the presence of the distribution of exchange interaction in the amorphous phase or high metalloid contents. (C) 2002 Elsevier Science B.V. All rights reserved.