The as-spun structure, thermal stability, crystallization structure and soft magnetic properties of Fe81.3SixB17-xCu1.7(x = 0-8) alloys were investigated. The Fe-Si-B-Cu amorphous alloys contain alpha-Fe nanocrystals with a high number density (N-d) in as-spun state and show uniform nanocrystalline structure and typical soft magnetic characteristics after annealing. The rise of Si content from 0 to 4 at% increases theN(d), while the further rise to 8 at% shows an adverse effect. The increasedN(d)enhances competitive growth between the crystals during crystallization process, then refines structure and improves soft magnetic properties of the nanocrystalline alloys. Contrarily, the decreasedN(d)results in coarsened nanostructure and deteriorated magnetic softness. The alloy with Si content of 4 at% contains alpha-Fe crystals with a highN(d)of 2.2 x 10(23) m(-3)in as-spun state and possesses fine alpha-Fe grains with an average size (D) of 14 nm, low coercivity (H-c) of 7.1 A/m, high effective permeability (at 1 kHz) of 16,500 and saturation magnetic flux density of 1.77 T after annealing at 668 K for 60 min. In addition, theH(c)of present Fe-Si-B-Cu nanocrystalline alloys is almost proportional toD(3)due to the high ratio of uniaxial anisotropy to average random anisotropy.