The present invention provides an organic thin film transistor (OTFT) being operatable at a low-voltage. The OTFT has a gate dielectric layer of ultra-thin metal oxide or a dual gate dielectric layer of metal oxide and organic dielectric. The metal oxide layer is self-grown to a thickness lower than 10 nm by direct oxidation of a metal gate electrode in O2 plasma process at a temperature lower than 100° C. The gate electrode is deposited with pattern on a plastic or glass substrate. An organic semiconductor layer is deposited on the gate dielectric layer, and source/drain electrodes are formed thereon. In case the dual gate dielectric layer is used, the source/drain electrodes can be disposed under the organic semiconductor layer to realize a bottom contact structure.