Dynamic viscoelasticity of poly(3-hexylthiophene) (P3HT) doped with iodine or FeCl3 under the influence of electric fields was studied. The doped P3HTs underwent a large decrease in elastic modulus under small electric fields on the order of 1 dc V/mm. The electric fields enhanced the loss tangent. The glass transition temperature decreased as the intensity of the applied fields was increased. This electroplastic behavior was observed also in ac excitation of less than 1 kHz. Tt was detected in both crystalline and amorphous P3HTs. Joule heating was a main process in the electroplastic behavior of doped P3HTs. We measured FTIR and X-ray diffraction spectra under electric fields to examine the possibility of other processes. The X-ray diffraction analysis suggested a possibility of another process caused by the formation of intra-planar packing of thiophene rings.