The structural and the magnetic properties of Fe58Co25Nb7Cu1B9 (M83) and Fe60Co25Nb6Cu1B8 (M85) alloys have been studied using differential scanning calorimetery (DSC), X-ray diffraction (XRD), transmission Mossbauer spectroscopy, and magnetic measurements. Thefirst crystallization peak temperatures (TX1) of the as-cast alloys M83 and M85 are 443 and 426 ℃ respectively, showing that a slight variation in composition reduces the stability of the alloy against crystallization. Mossbauer and XRD measurements reveal that crystallization of the specimens starts after annealing at 370 ℃, exhibiting the coexistence of a bcc crystalline phase and a residual amorphous matrix. Nanocrystallization leads to the formation of a bcc Fe-Co nanocrystalline phase consisting of 10 % Co and average grain diameters ranging between 10