Polymer-based thin film transistors （PTFTs） were successfully fabricated on silicon substrates which was used as gate electrode, thermal silicon dioxide was used as gate insulators and poly（3-hexylthiophene） as semiconducting active layers for the transistors. The fabrication and measurement of the devices were all performed in the clean air. The PTFTs with a surface-modified gate insulator show better electric characteristics with the field-effect mobility of 0.02 cm2/（Vs） and the on/off ratio higher than 105.