The process of a primary crystallization of the Fe72.5Cu1Nb2Mo2Si15.5B7 alloys was investigated by differential thermal analysis (DTA), x-ray diffraction (XRD) and transmission electron microscopy (TEM). Amorphous ribbons were isothermally annealed for 0.5, 2, 6, 30 and 150 minutes at 520 °C. Both, the XRD and TEM study showed that the level of devitrification of the sample increases with the annealing time. The above mentioned techniques confirmed the presence of the nanocrystalline grains of the Fe3Si phase and enable us to study the evolution of the identified phase. [ABSTRACT FROM AUTHOR]