The Langmuir-Blodgett (LB) technique was used to deposit a monolayer on the gate insulator to identify the role of the monolayer in the organic transistors. Ambipolar transistors were obtained on the basis of the CuPc LB monolayer and F16CuPc films. The results proved that a single molecular layer is enough to show field-effect performance. Moreover, the preorganized LB monolayer led to the alignment of subsequent molecules deposited on it through intermolecular pi-pi interactions.