In this work, as a new flexible gate insulator of organic thin film transistor (OTFT) device, PMDA-ODA-type polyimide (PI) were inserted between ITO and pentacene layer using modified ionized cluster beam (MICB) deposition method. The grain size of pentacene layers deposited by MICB method on ordered PI substrates was bigger than those by using conventional deposition method. The grain growth effect could be a good clue to explain increased mobility of our devices with MICB-deposited PI gate insulator. Since the flexibility and elasticity of polymeric gate insulator and conducting electrodes are essential for fabrication of flexible displays, the results of this work could be used for developing flexible all-organic devices and displays. (c) 2005 Published by Elsevier B.V.