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A thorough study about the influences of Mn substitution for Fe on the microstructure and magnetic characteristics of Fe73.5-xMnxSi13.5B9Nb3Cu1 (x3, 5) alloys prepared by the melt-spinning technique has been performed. Nanocomposites composed of nanoscale (Fe,Mn)3Si magnetic phase embedded in an amorphous matrix were obtained by annealing their amorphous alloys at 535oC for 1 hour. The addition of Mn causes a slight increase in the mean grain size. The Curie temperatures of the initial amorphous phase and of the nanocrystals phase decreased, while the Curie temperature of the remaining amorphous phase remained nearly constant with increasing Mn content. Soft magnetic properties of the crystallized samples have been significantly improved by a proper thermal treatment. Accordingly, the giant magnetoimpedance effect is observed and ascribed to the increase of the magnetic permeability, and the decrease of the coercivity of the samples. The increased magnetic permeability is resulted from a decrease in the magnetocrystalline anisotropy and saturation magnetostriction.