Improvement of dispersion of the Al2O3 nanoparticles in the poly(4-vinyl phenol) (PVP) matrix by coupling agent treatment resulted in a reduction of the leakage current density of the nanocomposite gate dielectric in organic thin-film transistor (OTFT) devices, which, in turn, improved the device performance compared to that of the device with the pure PVP gate dielectric. Under repetitive cyclic bending, the leakage current density of the nanocomposite layer was not changed, while that of the PVP layer was increased significantly. The nanocomposite gate dielectric layer provided the flexible OTFT device with improved mechanical and electrical stabilities.