The present study analyzed the effects of the polar functional groups and rough topography of the gate dielectric layer on the characteristics of pentacene field-effect transistors. For this purpose, prior to deposition of the organic semiconductor, we introduced polar functional groups and created a rough topography onto the poly(methylmethacrylate)/Al 2 O 3 gate dielectric layer using oxygen plasma treatment, and controlled the number of polar groups using an aging process. The mobility decrease observed after oxygen plasma treatment ranged from 0.2 to <0.01 cm 2 /V s and was related to the many polar functional groups and the rough topography of the gate dielectric, which formed localized trap states in the band gap and created disorder in the crystal structure. In addition, the electric dipole of the polar groups and the fixed interface charges induced a positive shift of the threshold voltage and an increase in the off-state current. After aging of the oxygen plasma-treated gate dielectrics, the reduced number of polar groups led to greatly enhanced charge mobility, a less positive shift of the threshold voltage, a lower off-state current, and lower activation energy compared to layers without aging. However, the mobility still remained lower than for layers without plasma treatment owing to the rough topography of the gate dielectric.