Organic thin film transistors were fabricated using pentacene and organic gate insulator on the glass substrates. Polyimide and photo-acryl were used as a gate insulator respectively. We investigated transfer and output characteristics of the thin film transistors(TFT's) having active layer of pentacene. We calculated field effect mobility and on/off ratio from transfer characteristics of pentacene TFT's, and measured IR absorption spectrum of polyimide used as gate insulator.