We investigated the effects of the surface wettability of two polymeric insulators in a 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) thin-film transistor (TFT). The chemical solvent of TIPS-pentacene was more wettable on a poly(4-vinylphenol) (PVP) insulator than on a poly(vinylidenefluoride-trifluoroethylene) (PVDF-TrFE) insulator. TIPS-pentacene molecules formed large crystals on the PVDF-TrFE layer, resulting in high field-effect mobility for the TFT. In contrast, for the PVP case, the mobility largely varied with the processing temperature of a TIPS-pentacene film. These results demonstrate that the surface wettability of an insulator is important for the crystallization of a solution-processed organic semiconductor and the resulting TFT performance.