The long-term stability of pentacene thin-film transistors (TFTs) encapsulated with a transparent SnO2 thin-film prepared by ion-beam-assisted deposition (IBAD) was investigated. When a buffer layer of 100-nm SnO2 film had been thermally deposited to reduce ion-induced damage prior to the IBAD process, our encapsulated organic thin-film transistors (OTFTs) showed somewhat degraded field-effect mobility of 0.5 cm2/V that was initially 0.62 cm2/V s, while the OTFTs without a buffer layer showed a 60% reduction in field-effect mobility after the IBAD process. However, surprisingly, the mobility was sustained up to one month and then gradually degraded down to 0.35 cm2/V s, which was still three times higher than that of the OTFT without any encapsulation layer after 100 days in air. The encapsulated OTFTs also exhibited superior on/off current ratio of over 105 to that of the unprotected devices (∼104), which was reduced from ∼106 before aging. Therefore, the enhanced long-term stability of our encapsulated OTFTs should be attributed to good protection of permeation against H2O into the devices with the IBAD SnO2 thin film, which was identified as having a dense amorphous microstructure with lots of OH groups. Passivation effects on the electrical properties of OTFTs are discussed in terms of the physical and chemical properties of the barrier films.