In this paper we report on thin film transistors based on poly(3-hexylthiophene) (P3HT) and carbon nanotubes (CNTs) composite active materials. Single walled CNTs were dispersed into P3HT chloroform solution. By drop casting, the composite solution was deposited onto the pre-fabricated device and formed thin active layer. The effect of different concentrations of CNTs to the charge carrier mobility of the composite was studied. Very little amount of CNT added into the active material can improve the carrier mobility while the on/off ratio is not reduced.