A potential profile along the channel in a conducting polymer field effect transistor (FET) has been measured directly using micro-manipulator with potential probing tip. The FET was fabricated with a head-tail coupled poly(3-hexylthiophene) as the channel semiconductor on a SiO2/n-Si substrate. The potential profile along the channel shows almost flat potential being equal to the source until near the drain. Then the channel potential abruptly increases near the drain, which has been observed for both linear and saturation regions in the drain current-drain voltage curves. The results are discussed in terms of carrier injection at the drain electrode and pinch off characteristics. (C) 2004 Elsevier B.V. All rights reserved.