Abstract: The effects of annealing temperature on nanocrystallization behavior and magnetic properties of amorphous Fe-Nb-B alloys were investigated by X-ray diffraction, differential scanning calorimeter and static magnetism measurement. It is found in experiment that the magnetic hardening takes place somewhat at the beginning of nanocrystallization process, which is manifested differently by the deterioration of coercive force and maximum permeability. The softer magnetic behavior features the alloys, of which the nanocrystallization structure is composed of α-Fe solid solution and amorphous matrix just before the precipitation of borides (e. g. Fe2B and Fe3B). The magnetic softening was observed increasing with the increase of the volume fraction of nanocrystallization phase, which can be explained by the reason that the decrease in the distance between nanocrystals will result in stronger exchange coupling.