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The effects of annealing and gamma irradiation on the magnetic properties of amorphous Co66Fe4B15Si15 alloys have been investigated by means of the changes in the giant magneto-impedance (GMI). The results show that there are no differences in the shape of the GMI curves for the samples which are as-quenched, annealed, gamma irradiated, and gamma irradiated after annealing. The GMI ratio showed drastic change according to the sample histories, the GMI ratio drastically increased in the annealed sample compared to as-quenched state, and this increase was recovered considerably by gamma irradiation, but no changes were observed in the as-quenched samples by gamma irradiation. These changes of GMI ratio with sample histories explained in terms of magnetic softness and the fluctuation of the circumferential/transverse anisotropy by gamma heating.