Three nanocrystalline alloys, Fe75−xCox(Ni70Zr30)15B10 (x = 0, 10, and 20), were synthesized from elemental powders in a planetary high-energy ball mill. Their microstructure, magnetic properties, and thermal stability were characterized by X-ray diffraction, transmission Mössbauer spectroscopy, transmission electron microscopy, scanning electron microscopy, induction coupled plasma, vibrating sample magnetometry, and differential scanning calorimetry. After 80 h of milling, the nanocrystallites size of alloys is in the range 6–10 ± 1 nm. The lattice parameter decreases when increasing (decreasing) milling time (Fe content). Furthermore, the thermal stability of the nanocrystalline phase increases when increasing Co concentration. The activation energy of the main crystallization process, between 275 ± 8 and 311 ± 10 kJ mol−1, is associated with grain growth. Slight contamination from milling tools and milling atmosphere was detected. Minor differences were detected after Mössbauer analysis.