Low operation voltage organic thin film transistors (OTFTs) were successfully fabricated with top contact structure at low fabrication temperature. The thin gate dielectric of OTFTs were deposited by atmospheric pressure plasma jet at the substrate temperature about 150°C and under atmospheric pressure. The environment of processes would significantly improve the abilities of large area application for display and decrease the cost of instruments. We found that the quality of silicon oxide deposited with atmospheric pressure plasma jet strongly depended on the main gas and the gap distance between the plasma nozzle and the surface of the device and even influenced the deposition rate. Due to the improvement of gate insulator quality, good electrical characteristics of OTFTs can be obtained, such as carrier mobility as large as 0.66 cm2/ V·s, operation voltage as low as -2 V, and subthreshold swing as low as 0.7 V/dec.