Crystallization behavior of the amorphous phase in the Co–Fe–B–Si alloy system was investigated by X-ray diffraction technique under the influence both of thermal and irradiation activation. Crystallization starting temperature decreases by 200–300 ° C under the influence of the irradiation. After partial crystallization of the amorphous alloy under the action of the irradiation annealed below the thermal crystallization temperature, the amorphous phase was again formed while annealing the alloy without irradiation. During irradiation and thermal annealing formation of the new metastable phase has been discovered with the diffraction pattern similar to the previously discovered cubic aperiodical phase in the Fe–Nb–Cu–B–Si alloy.