Structural properties and influence of thermal treatment on electrical properties for organic thin-film transistor (OTFT) with 3nm-thickness active layer pentacene were investigated. The pentacene film was preferentially oriented to (00l) of thin-film phase and carriers conducted thought standing molecules slantingly. The carrier mobility of the ultrathin OTFT was decreased, and the threshold voltage was shifted to negative direction by thermal treatment. For the ultrathin OTFT, the carrier conduction was affected by H2O and O-2 because of change in the interface state and the hole localization state by adsorption of O-2 and H2O and oxidation.