Pentacene films vacuum-sublimed on a cross-linked polymeric insulator (CPVP-C6) prepared using poly(4-vinylphenol) (PVP) and 1,6-bis(trichlorosilyl)hexane (C6) were studied with a special concern on possible influences of the CPVP-C6 thickness on the electrical characteristics of the pentacene films. It was found that the conductivities of the pentacene films on a thin CPVP-C6 film (10 nm) were approximately 2 orders of magnitude higher than those on a glass substrate and increased slightly with the increase in the thickness of the underlying CPVP-C6 film. In addition, the X-ray diffraction measurements showed that the stacking structure of pentacene molecules was remarkably enhanced by increasing the thickness of the CPVP-C6 film, suggesting that the increase in conductivity is due, at least in part, to the improvement in carrier mobilities caused by the growth of large pentacene grains. An attempt to directly evaluate carrier mobilities using pentacene/CPVP-C6 field-effect transistors was made, and a seeming increase in the carrier mobilities observed with the increase in the CPVP-C6 thickness was ascribed to a hygroscopic nature of the CPVP-C6 film, which was evidenced by the capacitance and quartz crystal microbalance measurements. Possible reasons are discussed to explain the enhanced conductivities of the pentacene films on the increased thicknesses of CPVP-C6.