Poly(3-hexylthiophene)(P3HT) transistors with a thin ion-gel gate dielectric layer (100 nm thickness) was fabricated. The thin ion-gel dielectric layer retarded the capacitance drop at high frequencies and the diffusion of the ionic molecules in the polymer active layer that are severe drawbacks of the ion-gel dielectric transistors. Thereby, the thin ion-gel transistors showed hysteresis-free I-V characteristics, less frequency-dependence, and enhanced bias-stability. The average charge mobility was similar to 2 cm(2)/Vs and the on/off ratio was 10(4)similar to 10(5). The dependence of the capacitance and the kinetics of ion translation on the thickness of the ion-gel were discussed by both experiments and theoretical calculations.