Soluble conjugated polymers such as poly(3-hexylthiophene) (P3HT) are very promising candidates for a cheap electronic device on various substrates. In this study we report the effects of O2 plasma treatment of the substrates on the electrical properties of P3HT organic thin-film transistors and metal/insulator/organic semiconductor capacitors. Based on the results of a capacitance–voltage measurement, the effective charge density in the interface between P3HT and SiO2 layer treated by O2 plasma for 30 s was approximately -18.3 nC/cm2. When the period of O2 plasma treatment was longer than 30 s, the field-effect mobility decreased since the amount of charge and the relaxation time constant of interface traps increased. © 2003 American Institute of Physics.