We report on the surface-induced molecular order and the optical anisotropy of a pentacene thin film on a photo-aligned polymeric gate insulator layer. Since a pentacene molecule has a rodlike shape, the induced orientational order of the pentacene molecules is expected on an alignment layer for liquid crystal molecules. The photo-aligned polymer insulator, exposed to a linearly polarized ultraviolet light, has a high cross-linked density and thus induces the structural order of the pentacene molecules. The pentacene thin film deposited on the photo-aligned polymeric insulator is found to show the optical anisotropy. The organic thin film transistor on the photo-aligned insulator exhibits good electrical properties.