In this work, we report on the study of pentacene field effect transistors with a polymer (PMMA) on a high-k oxide (Ta<sub>2</sub>O<sub>5</sub>) two-layer gate dielectric, in order to combine low voltage operating devices and stability. Two-layer dielectric devices were compared to single layer (Ta<sub>2</sub>O<sub>5</sub> or PMMA) gate insulators. To assess the importance of the structural quality of the Ta<sub>2</sub>O<sub>5</sub>, two deposition processes (e-beam evaporation and anodization) are studied. In order to evidence the relation between the electrical performances and behavior and the structural quality of the organic thin film, X-ray diffraction and AFM measurements were carried out to determine the pentacene morphology and ordering on the different dielectrics. Devices with bilayer gate dielectric present the best mobility, up to 0,6cm²/V/s, the pentacene film being well ordered in large grains on PMMA with or without high-k under layer. The transistors operate at 15V and show reduced polarisation effect compared to devices with only Ta<sub>2</sub>O<sub>5</sub>.