With the aim of enhancing the field-effect mobility of poly(3-hexylthiophene) (P3HT) field-effect transistors (FETs), we added functionalized multiwalled carbon nanotubes (CNTs) to the P3HT solution prior to film formation. The nanotubes were found to be homogeneously dispersed in the P3HT films because of their functional groups. We found that at the appropriate CNT concentration (up to 10 wt% CNT), the P3HT FETs have a high field-effect mobility of 0.04 cm 2 V À1 s À1 , which is an improvement by a factor of more than 10. This remarkable increase in the field-effect mobil-ity over that of the pristine P3HT film is due to the high conductivity of the CNTs which act as conducting bridges between the crystalline regions of the P3HT film, and the reduction in the hole-injection barrier due to the low work function of CNTs, which results in more efficient carrier injection.