A zone annealing process is adopted to improve the performance ofpentacene organic thin film transistors. The process is completed in a glove box full of dry nitrogen gas and with a heating wire (φ0.1 mm tungsten wire) inside.The device have the structure of glass/ITO (150 nm)/Ta2 O5 (600 nm)/pentacene (60 nm)/Au (20 nm). Under thegate-source voltage -60 V and the drain-source voltage -60 V, the source-drain current, the mobility, and theon/off current ratio increase from 4 × 10-7A to 2.0 × 10-6 A, 2.9 × 10-3 cm2V-1s-1 to 1.6 × 10-2 cm2 V-1s-1,and 2 × 103 to 5.33 × 103, respectively. It is found that the distance between the pentacene grain boundaries decreases and an obvious layer structure can be formed in grains after annealing. According to the transport of carriers in the polycrystalline film, these changes of the pentacene film can improve the source-drain current and the mobility.