A novel type of absorber integrate into a standard WR-10 metal waveguide of is developed and measured. The absorber is based on alumina nanofibers covered with single or multi- carbon layers. Employing this technique for the CNT based absorbers offers a material with micron scales-oriented 3D microstructures, that is hybrid alumina nanofibers covered with carbon layer. These microstructures result in a low level of reflectance and good absorbance at 75-110 GHz frequency band due to the highly porous and low conductivity.