The influence of Mn content on the structure and magnetic permeability of Nd3Fe68-xMnxCo18B11 (x = 0, 1, 2) amorphous soft magnetic alloys was studied. The ribbons were prepared by melt-spining. The permittivity–frequency and permeability–frequency properties were determined in the microwave frequency regime of 2-18 GHz by vector network analysis. XRD spectra showed that only α-Fe diffraction peak was observed in the as-spun alloy. It is found that the acquired complex permittivity and permeability values match the microwave frequency when the 1at% Mn content doped. The results suggest a new design of microwave absorbers based on electromagnetic wave absorbing materials.