Poly-3-hexylthiophene (P3HT) based thin-film transistors were fabricated. Various spin-coating conditions including spin speeds, curing temperatures, solvents, and weight percentages of P3HT have been examined. It was found that the P3HT of 0.3 wt % dissolved in chloroform contributes to the lowest surface roughness and the highest ON/OFF current ratio over four orders of magnitude. Besides, the anomalous gate leakage current was eliminated successfully by growing a thick field oxide layer in the source/drain regions. Through O-2, N-2, H2O, or high-vacuum treatments, it was confirmed that the threshold voltage and mobility of organic devices degrades rapidly when the P3HT-based organic thin-film transistors were exposed to the O-2 atmosphere within 8 h. Moreover, the threshold voltage increases under a positive gate bias stress while the V-th decreases after a negative gate bias stress. The mechanism was attributed to the polarization effect of P3HT film. (C) 2004 The Electrochemical Society.