The paper deals with the combined influence of the thermal nanocrystallization temperature and time and the milling time on the properties of the nanocrystallization process and the structure of powders based on METGLAS (metallic glass). Thermal treatment of Fe₇₈Si₁₃B₉ metallic glass was carried out for 1h at the temperature range of 100–600°C with 50°C temperature steps. The heat treated material was then processed by high-energy milling. It was shown, that the use of a combination of thermal nanocrystallization and high-energy milling produced metallic glass-based nanocrystalline powders containing Fe-α, Fe₃B and Fe₂B crystallites in the amorphous matrix. Then, the nanocrystalline powders were used to construct composite toroidal cores.