Abstract The microstructure and soft magnetic properties, i.e. saturation induction, remanence, coercivity and permeability of high-energy ball milled nanostructured FINEMET powder cores and nanocomposite cores with average powder particles diameter between 25 and 75 μm, 300 and 500 μm and 750 and 1200 μm are presented in this paper. These results showed that the permeability of the powder and composite cores increases if the particle size increases. It was shown that the use of a combination of thermal nanocrystallization and high-energy milling produced metallic glass-based nanocrystalline powders containing α-Fe(Si) crystallites in the amorphous matrix. It was also found that the magnetic properties of the composite cores could be controlled in a wide range depending on powder particle size and content of polymer binder.