A new (Fe0.76Si0.09B0.1P0.05)98.25Nb1Cu0.75 bulk nanocrystalline alloy in a rod shape with a diameter of 2.5 mm was successfully developed by copper mold casting and isothermal annealing. It was found that the introduction of Cu can result in the formation of a large number ofα-Fe(Si) clusters (less than 3 nm) embedded in the glassy matrix, which greatly improved the primary crystallization of α-Fe(Si) nanocrystals during subsequent annealing. The obtained superior soft magnetic properties can be ascribed to the formation of optimal nanocomposite structures with a large number of α-Fe(Si) nanocrystals ( ? 17 nm) uniformly distributed in the glassy matrix.