The electrical properties of Organic thin-film transistor (OTFT) with a pentacene active layer of 3nm and 50nm were examined. By a new estimation method of applied voltage in channel layer of OTFT, it was found that the ratio of potential drop to lateral direction of channel at on-state is smaller than that at off-state and also that of 3nm thickness OTFT is larger than 50nm thickness. The on-state current of 3nm thickness OTFT was larger than that of 50nm thickness. The main reason of this phenomenon is due to the difference of the resistance of hole injection from Au to pentacene. The quantum-mechanical effect is also discussed.