The peculiarities of the formation of a nanocrystalline structure and induced magnetic anisotropy have been studied in Fe87−xCu1Nb3SixB9 alloys (x = 0, 4, 6, 8, 9.5, 11, 13.5 at. %) after thermomechanical treatment. Differences in the dynamics of the process of phase separation in the initial amorphous state, final phase composition, and magnitude and sign of induced magnetic anisotropy have been revealed depending on the initial content of silicon.