We present the effect of poly (3-hexylthiophene) (P3HT) thickness on the performance of organic thin film transistors (OTFTs). The P3HT film thickness varies from 11 to 186 nm. The devices have channel lengths of 5, 10, 20, 40, and 80 Am and a channel width of 500 Am. The mobility and on/off ratio are up to 0.08 cm(2)/V s and 7 X 10(3), respectively. The drain current and the mobility increase with thickness. At the same P3HT thickness, the drain current and mobility become higher when the channel length is reduced. The on/off ratio decreases quickly and then saturates for thickness > 64 nm. Short channel devices have higher on/off ratio than long channel devices. For short channel devices (5 mu m), the on/off ratio does not change significantly with thickness. The devices with shorter channel length and thicker P3HT films tend to have smaller threshold voltages. The threshold voltage saturates for long channel (20-80 mu m) devices, for films thicker than 110 nm. The gate leakage (ID offset) is higher for thicker film devices. The performance dependence as a function of P3HT film thickness can be explained by the bulk conductance model and the SiO2 surface potential change. Our results suggest that the performance of P3HT OTFTs should be optimized based on the specific application. (c) 2006 American Vacuum Society.