Microwave absorption has attracted a considerable attention in the last decade. Various factors have effect on the microwave attenuation such as permittivity and permeability of absorbers. In this research, these properties were provided by multiwall carbon nanotube (MWCNT) as a conductive polymer and Zn_0.25Co_0.75Fe₂O₄ as a magnetic nanoparticle. MWCNTs were functionalized with carboxylic acid groups through the sonochemical method by the mixture of nitric and sulfuric acid, due to their better dispersion in the medium reaction and enhancing interfacial polarization, and then magnetic nanoparticles were formed base on the functionalized MWCNTs through the solvothermal method by use of ethylene glycol as a solvent. Finally, MWCNT/Zn_0.25Co_0.75Fe₂O₄ nanocomposite was blended in the silicone rubber as a polymeric matrix to investigation of microwave absorption properties. Zn_0.25Co_0.75Fe₂O₄ nanoparticles and MWCNT/Zn_0.25Co_0.75Fe₂O₄ nanocomposite were identified by the diffuse reflection spectroscopy (DRS), Fourier transform infrared (FT-IR), field emission scanning electron microscopy (FE-SEM), and investigation of microwave absorption properties was performed by vector network analyzer (VNA). Results indicated that magnetic nanoparticles and magnetic and dielectric MWCNT/Zn_0.25Co_0.75Fe₂O₄ nanocomposite have been prepared and absorbed more than 47% of microwave at x-band. Moreover, maximum reflection loss of this nanocomposite was 15 dB at 11.96 GHz.