Specific features are revealed of how composite materials are formed on the basis of polyethylene and multi-walled carbon nanotubes modified with iron oxide nanoparticles (Fe 3 O 4 /MWCNT–PE), produced by the mechanical mixing method from a polyethylene melt. The conditions in which the composite materials are obtained were optimized to provide a uniform distribution of the Fe 3 O 4 /MWCNT filler in the polyethylene matrix. The influence exerted by the Fe 3 O 4 /MWCNT filler on the electrical properties of the resulting composite materials was determined. Introduction of Fe 3 O 4 nanoparticles gives rise to magnetic properties of a composite material in the frequency range from 1 kHz to 2 MHz. An analysis of the frequency dependences of reflectance, transmittance and absorbance demonstrated that an increase in the sample thickness leads to a higher reflectance and lower transmittance. The composite materials can be used to create coatings lowering the electromagnetic radiation intensity by up to 40%. It was shown that increasing the content of Fe 3 O 4 /MWCNT to more than 10 wt % leads to a decrease in both the electrical conductivity and the complex dielectric permittivity and magnetic permeability of the composite material. This occurs due to the decrease in the flowability of the polymer material and to the resulting nonuniform distribution of the filler in the bulk of polyethylene.