The amorphous Bi5Nb3O15 film grown at room temperature under all oxygen-plasma sputtering ambient (BNRT-O-2 film) has a hydrophobic surface with a surface energy of 35.6 mJ m(-2), which is close to that of the orthorhombic pentacene (38 mJ m(-2)), resulting in the formation of a good pentacene layer without the introduction of all additional polymer layer. This film was very flexible, maintaining a high capacitance of 145 nF cm(-2) during and after 10(5) bending cycles with a small curvature radius of 7.5 mill. This film was optically transparent. Furthermore, the flexible, pentacene-based, organic thin-film transistors (OTFTs) fabricated oil the poly(ether sulfone) substrate at room temperature using a BNRT-O-2 film as a gate insulator exhibited a promising device performance with a high field effect mobility of 0.5 cm(2) V-1 s(-1), an on/off current modulation of 10(5), and a small subthreshold slope of 0.2 V decade(-1) under a low operating voltage of -5 V. This device also maintained a high carrier mobility of 0.45 cm(2) V-1 s(-1) during the bending with a small curvature radius of 9 mm. Therefore, the BNRT-O-2 film is considered a promising material for the gate insulator of the flexible, pentacene-based OTFT.