We demonstrate a dimethyl ketone treatment for structural modification of cross-linked poly(4-vinylphenol) (c-PVP) gate insulator in pentacene thin-film transistors (TFTs). Here, the dimethyl ketone treatment was performed immediately after spin-coating the insulator solution, followed by thermal annealing for cross-linking of PVP molecules. It is found that the dimethyl ketone treatment lowered the surface energy of the PVP gate insulator and made the PVP film thinner. The results of X-ray diffraction and atomic force microscopy analyses showed that the dimethyl ketone-treated c-PVP insulator contributes to enhancing the crystallinity of pentacene semiconductor films and reducing the density of lamellar grains and bulk phase crystallites in pentacene films. Consequently, the performance improvement of the pentacene TFT is achieved by structurally modifying the c-PVP gate insulator through the dimethyl ketone treatment.