The crystallization and magnetic properties of Fe72.5Nb4.5Cu1Si10B12, Fe73.5Nb3Cu1Si13.5B9 and Fe72Nb4.5Cu1Si13.5B9 alloys have been studied. After primary crystallization, the α-Fe(Si) phase was formed for the alloy which has 10 at.% Si. The ordered Fe80Si20 phase was formed for alloys containing 13.5 at.% Si. The activation energy for the formation of the α-Fe(Si) phase was 67 kcal mol1, which is lower than that for the formation of the ordered phase. In the scannins temperature variation or resistivity measurement, a drop in resistivity was found during the formation of the α-Fe(Si) phase whereas a rise in resistivity was observed during primary crystallization for the other two alloys. Grain-boundary scattering and the low-temperature coefficient of resistivity were the causes of the rise in resistivity during the formation of the ordered phase. The Fe72Nb45Cu1Si13.5B9 alloy exhibited superior soft-magnetic properties among the measured alloys when annealed between 875 and 875 K for 30 min. The rapid deterioration in the magnetic properties at higher temperatures of the annealed alloys was primarily due to the formation of the Fe2B phase, which has a strong magnetocrystalline anisotropy energy. © 1998 Taylor & Francis Group, LLC.