In this paper, it was demonstrated that pentacene thin-film transistors (TFTs) were fabricated with an organic adhesion layer between an organic semiconductor and a gate insulator. In order to form polymeric film as an adhesion layer, a vapor deposition polymerization (VDP) process was introduced to substitute for the usual spin-coating process. Field effect mobility, threshold voltage, and on/off current ratio in pentacene TFTs with a 15 nm thick organic adhesion layer were about 0.4 cm2/Vs, -1 V, and 10(6), respectively. We also demonstrated that threshold voltage strongly depends on the stress time when a gate voltage has been applied for bias stress test. We suggest that a polyimide adhesion layer fabricated by the VDP method can be applied to realize organic TFTs with long-term stability because of lower threshold voltage shifts due to reduced charge trapping at the interface between the pentacene semiconductor and the polyimide layer.