Abstract: The nanocrystalline structure of Fe_73.5Cu₁Mo₃Si_13.5B₉ alloy annealed at 520℃ for 20-120 minhas been investigated by using XRD and TEM methods. The crystalline phase of this annealed alloy is α-Fe(Si) which has a size of 14.0nm and consists of the DO₃ ordered region and disordered region. The volume fraction, Si content and the size of the DO₃ ordered region of α-Fe(Si)phase increase as the annealing time increases. When the annealing time reaches 60 min, the residual amorphous phase is in a special structural state. Both α-Fe(Si) phase and the residual amorphous phase may affect the initial permeability of nanocrystalline Fe_73.5Cu₁Mo₃Si_13.5B₉ alloy.