Effects of oxygen plasma generated in a sputtering process for deposition of electrodes on pentacene thin films to configure top-contact (TC) transistors have been thoroughly investigated. Reactive oxygen species severely degraded electrical properties of pentacene films during the deposition of RuOx electrodes, leading to a failure of devices. In the off-region, the leakage current increased by about two orders of magnitude, and the subthreshold slope also increased by 6.5 times. The top surface of pentacene films was oxidized by oxygen plasma and C-O and C=O bonds awerere created. The pentacenequinone derivative was confirmed by X-ray photoelectron spectroscopy. The oxidation of pentacene films gives rise to charge traps at the pentacene/electrode interface, which produces a leakage channel between source and drain electrodes. We believe that this side effect of oxygen plasma on the fabrication of TC-devices should be considered carefully.