This paper deals with the investigation results of nanocomposite permittivity based on magnetite nanoparticles and a polymeric matrix in electromagnetic field. Dependencies of real and imaginary parts of nanocomposite permittivity on the frequency of the electromagnetic field and size and concentration of magnetite nanoparticles have been found, and the dependence of imaginary part of permittivity on the real one has been determined. The Cole–Cole type has the dependence of the imaginary part of permittivity on the real one at high frequencies of electromagnetic field (at small values of the real part of permittivity) and hyperbolic behavior at small frequencies of the electromagnetic field (at large values of the real part of permittivity). Refraction index, loss angle tangent, and penetration depth of the nanocomposite. [ABSTRACT FROM AUTHOR]