We report on the fabrication of flexible pentacene thin-film transistors (TFTs) encapsulated with an ultraviolet (UV)-protecting SnO2 thin-film that has been prepared by ion-beam-assisted deposition (IBAD). We deposited thermally evaporated SnO2 on a pentacene TFT as a buffer layer prior to the IBAD process. When a UV light of 254 nm wavelength was continuously illuminated onto our encapsulated device for periods of 5, 30, 60, and 120 min under a vacuum of 1x10(-6) Torr, its field mobility gradually degraded from 0.34 to 0.22 cm(2)/V s in 2 h while the other device without encapsulation rapidly degraded (its mobility was reduced to 0.021 cm(2)/V s during the same period). Our X-ray diffraction data indicates that the UV-induced-degradation of device characteristics is directly correlated to the degradation of pentacene crystallinity against UV radiation. We conclude that our IBAD SnO2 encapsulation is a promising way to protect pentacene TFTs. (c) 2006 The Electrochemical Society.