Poly-3-hexylthiophene (P3HT) is coated on a chemically modified SiO 2 gate dielectric layer as an active layer to fabricate organic thin-film transistors (OTFTs). This study uses a new atmospheric-pressure plasma (APP) approach to treat SiO2 surfaces, and investigates its effect on the electric characteristics of OTFTs. The APP process can be performed at low temperature (<1007:) and atmospheric pressure, so it is very suitable for use on a plastic substrate. After the SiO2 surface has been modified by the APP process with hexamethyldisilazane (HMDS), it exhibits typical I-V characteristics of TFTs. Calculations reveal that its field effect mobility can reach 0.02-0.03 cm2/Vs, which is about 15 times higher than that before the modification, and the threshold voltage is below -10V. The performance is even better than that obtained following the usual surface treatment of the SiO2 surface by spin-coating or evaporation.