Polycarbonate - carbon nanofibers, containing various concentrations of filler ranging from 0 % to 10 % wt. have been investigated by electron spin resonance. The as obtained electron spin resonance spectra are complex due to the overlap of several resonances originating from the conducting electrons delocalized over the metallic domains of nanotubes, from the free radicals generated within the polymeric matrix during the processing step, from insulating graphite particles, and from catalyst residues (observed as magnetic impurities). The effect of the filler on the resonance line shape is analyzed in detail. The shape of resonance spectrum demonstrates that the resonance line located near g=2.00 originates from conducting electrons delocalized over the metallic domains of nanotubes. Thos line was accurately fitted by a Lorentzian line shape.