Organic thin film transistors (OTFTs) have attracted considerable attention because of their potential to be utilized as electronic switching elements in flexible and large area devices.In order to utilize these devices in reality, the stability of devices, especially the bias stress induced shift of threshold voltage (VT), namely bias-stress effect, should be controlled well.The shift of threshold voltage is attributed to slow trapping in and near the channel.It is well known that the channel formed in very few layers close to the insulator layer.Thus, it is meaningful to consider the influence of the characteristics of the first pentacene layer on the bias-stress effect.