We have examined electronic and structural interactions at the interfaces of organic semiconductors and their insulation layers or electrodes with regard to organic electric field effect transistors. It was revealed that the crystal structure of an organic semiconductor layer depends on its preparation conditions and the surface energy of the substrate, especially near field region of the interface. Also it was revealed that the control of the surface smoothness of the insulator layer, contact condition between the organic semiconductor layer and the insulator layer or electrode are essential to obtain an excellent FET characteristic.