From the view point of materials sciences, one of the central issues in organic thin film transistors (TFTs) is the interface between different materials inherent in the device structure. For example, the interface between organic semiconductors and electrodes controls the carrier injection, while the interface between organic semiconductors and gate insulators governs the trap and carrier densities. Here, we show that interface modification with self-assembeld monolayers (SAMs) using polar organosilane molecules offers novel functions in organic TFTs. SAMs on SiO2 gate dielectrics was found to the carrier density at the conduction channel, while the adsorbed SAMs molecules on metal electrodes causes an ambipolar operation in fullerene TFTs. These interface modification techniques, since they are low temperature processes, provide novel opportunities for improving device manufacturing processes.