Values of the effective constants of local magnetic anisotropy and sizes of the regions of correlation of the magnetic anisotropy axes of the amorphous and crystalline phases in the Fe73.5Cu1Nb3Si13.5B9 alloy were determined, and their variation with annealing temperature was studied. Since the above parameters are closely related to microstructure, the data obtained enable one to judge the variation of the microstructure with the annealing temperature. The initial stages of the alloy crystallization were studied. The important role of the interfacial amorphous phase in the formation of high soft magnetic properties of the nanocrystalline alloy is emphasized. The results of magnetic measurements are compared with X-ray diffraction data from which Si content in the α-FeSi phase was determined and its grain size was evaluated.