The effects of the surface roughness of the nanocomposite gate dielectric on the correlation between grain size and mobility of pentacene organic thin-film transistors were investigated. In this work, the nanocomposite as gate dielectric film was carried out by blending polyimide and nano-TiO2 particles to enhance the capacitance of the gate dielectric. The roughness of the nanocomposite gate dielectric is varied from 0.8 to 20 nm with increasing TiO2 concentration from 0 to 5 vol % in polyimide. Grain size of pentacene decreases with increasing roughness of the nanocomposite gate dielectric film. Two distinguishable regions are observed on the correlation between grain size and mobility of pentacene. In region (I), the nanocomposite gate dielectric with 0-2 vol % TiO2 content shows that mobility does not significant change with decreasing grain size of pentacene, similar to the behavior of the organic polymer gate dielectric. In region (II), the nanocomposite gate dielectric with 3-5 vol % TiO2 content shows that mobility is remarkably decreased with decreasing grain size of pentacene, similar to the behavior of the inorganic gate dielectric.