Performances of organic thin-film transistors (TFTs) can be detrimentally affected by the state of gate dielectric layer. In this study, we report the condensation and annealing effects of a polymer gate dielectric layer. For the observations of the effect of condensation, the spin-coated polymer layers with various deposition conditions were fabricated and kept under low vacuum condition for several days. It is observed that the thickness of polymer layer and the electrical characteristics of organic TFTs vary with the condensation time, and the appropriate additional annealing process could stabilize the gate dielectric layer and even improve the device characteristics consequently.