The fabrication of poly(3-hexylthiophene)(P3HT)-based field-effect transistors (FETs) on plastic substrates by solution processes has been systematically investigated. Top-contact P3HT FETs with polycarbonate (PC) substrates are fabricated using organic-inorganic hybrid gate insulators of poly(methylsilsesquioxane) (PMSQ) with a low curing temperature. It is found that the electrical characteristics of fabricated P3HT FETs are significantly influenced by residual oxygen and/or water in PC substrates and the drain current is remarkably increased even in oxygen and water-free environments. The electrical performances of the FETs on PC substrates are recovered by thermal annealing under vacuum and the annealed FETs exhibit stable characteristics, which are comparable to those of the FETs fabricated on glass substrates with PMSQ gate insulators. (C) 2008 Elsevier B.V. All rights reserved.