This investigation deals with the synthesis and detailed study of a photoinitiator-free photosensitive polyimide gate insulator for organic thin-film transistors (OTFTs), one of the most important components of active-matrix displays on plastic substrates. The photosensitive polyimide precursor poly(amic acid) is prepared from the aromatic dianhydride 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA) and the novel aromatic diamine 7-(3,5-diaminobenzoyloxy)coumarine (DACM). The photosensitivity of the poly(amic acid) film is investigated using a high-pressure mercury lamp at 280–310 nm. The pattern resolution of the photocured film was about 50 μm. The surface morphology of the films before and after the photopatterning process is also investigated. In addition, we have fabricated pentacene OTFTs with the photoinitiator-free photosensitive polyimide as gate insulator. The OTFT characteristics are discussed in more detail with respect to the electrical properties of the photosensitive polyimide thin film.