The device instability phenomena have been studied from the viewpoints of the molecular structural ordering and the interface state modification in pentacene field-effect transistors. Here, we provide clear evidences that bias-induced threshold voltage shift (Delta V-T) and hysteresis predominantly come from the charge trapping in the pentacene/SiO2 interface region. An increase of the external gate bias stress accelerates the charge trapping into the deeper states of the forbidden gap of pentacene, resulting in a further shift of V-T. The gate bias dependence of Delta V-T also reveals that the trapping phenomenon is strongly influenced by the molecular structural ordering, particularly adjacent to the pentacene/SiO2 interface. Furthermore, we show an effect of the chemical modification of SiO2 surface on reducing Delta V-T. The dielectric surface with alkyl chain functional groups leads to a highly ordered molecule structure in the channel thanks to a reduction of the surface energy, which would suppress the trapping site formation in the film as well as at the interface. (C) 2008 The Electrochemical Society.