Polyaniline (PANI) nanofibers doped with hydrochloric acid were prepared by “rapid mixing” of aniline monomer and oxidant. The dedoped PANI nanofibers were obtained by dedoped using ammonium hydroxide. The morphology of PANI was characterized by scanning electron microscopy (SEM). The dedoped PANI nanofibers/low density polyethylene (PANI/LDPE) composites with different PANI nanofibers contents were obtained by melt blending method. The structures of the nanocomposites were studied by Fourier transform infrared spectrometer (FT-IR). Electrical conductivity, breakdown strength under different temperatures, the relative dielectric constant and the dielectric loss at different frequencies of dedoped PANI nanofibers/LDPE composites were measured. The results show that the dedoped PANI nanofibers are uniform with diameters; the electrical conductivity of the nanocomposites with a modest amount of dedoped PANI nanofibers decreases; the breakdown strength of the nanocomposites does not reduce with a small amount of dedoped PANI nanofibers; the relative dielectric constant of the nanocomposites declines first and then increases, but the dielectric loss factor increases all the time.