The effects of annealing and gamma irradiation on the magnetic properties of amorphous Co66Fe4B15Si15 alloys have been investigated by means of the changes of the permeability spectra. In general, there are no differences in the magnitude and shape of the real part of the permeability curves between as-quenched amorphous and gamma-irradiated samples. However, a drastic increase, caused by gamma irradiation, is observed in the magnitude of the imaginary part of the permeability. This can be attributed to another relaxation process by superposition due to the gamma-irradiation effect. The same trend was also found in the sample annealed at 380°C in vacuum and in open air under a field of 3 Oe, which has a maximum value of the permeability due to the presence of ultrasoft magnetic materials.