In this work the improvement of an efficiency of electrostatic harvesters by using polymer/nanostructured carbon composites (PNC) is investigated. In electrostatic harvesters the energy of mechanical oscillations is conversed into electrical energy through deformation of an insulator (dielectric) layer between plates of a charged capacitor. To increase the change of voltage during the deformation of dielectric layer between capacitor plates, its dielectric permittivity should decrease with tensile deformation. Capacitor samples containing PNC dielectric layer were elaborated from polyisoprene, electroconductive carbon nanoparticles and brass electrodes.Tensile and compressive deformation measurements conducted on PNC samples showed, that under tensile and compressive deformation dielectric permittivity of the PNC composite is decreasing. It was also found, that the character of dielectric permittivity changes for both - tensile and compressive deformation - is different. The change of dielectric properties observed indicates about complex processes taking place in the PNC dielectric layer.