We fabricated perylene-based organic thin-film transistor (OTFT) with an octadecyltrichlorosilane (OTS) monolayer in an ultrahigh vacuum condition. By current-voltage characteristics, the saturation current and the field effect mobility of OTS-OTFT (gold/perylene/OTS/SiO2/p(+)-Si) increased over similar to 100 times in comparison with normal-OTFT (gold/perylene/SiO2/p(+)-Si), and on/off ratio increased over similar to 100 times. The saturation current, field effect mobility, and on/off ratio were > 1 mu A, 0.042 cm(2)/V s, and > 10(6), respectively. The uniformity of the OTS layer was confirmed by x-ray reflectivity. Perylene thin films on SiO2 and OTS/SiO2 were compared by atomic force microscopy, scanning electron microscopy, x-ray diffraction, and x-ray photoscopy. (c) 2005 American Vacuum Society.