An extra electric field is introduced below the spinneret in an electrospinning setup for producing poly(3-hexylthiophene) (P3HT) nanofibers. The liquid jet is greatly prolonged by the additional extensional force and thinner fibers can thus be obtained. The chain conformation and orientation in fibers are probed by differential scanning calorimetry and X-ray diffraction techniques. Under the influence of the secondary electric field, P3HT chains are extensively stretched and aligned along the fiber axis. The electrospun P3HT nanofibers are fabricated into field-effect transistors and the charge carrier mobilities of the nanofibers with and without secondary electric field are found to be 1.54 x 10(-4) and 1.62 x 10(-1) cm(2) V-1 s(-1), respectively. The dramatic enhancement of mobility by more than 1000 times is due to the effective charge transport through the delocalization of electrons along the highly extended and oriented P3HT backbones rather than the ordinary pi-pi stacking. In addition to P3HT, it is found that this simple method also works for poly(3-alkylthiophene).