We investigated interfacial roughness effect of pentacene-based organic thin film transistors on an anisotropic insulator fabricated by obliquely evaporated silicon dioxide. It was observed that the obliquely evaporated gate dielectric layer affected molecular ordering of evaporated pentacene molecules. As the evaporation angle of SiO2 increased, the anisotropic interaction at the dielectric interface and the ordering of organic semiconductor molecules increased. We obtained the anisotropic ratio of the field-effect mobility for carriers transported parallel and perpendicular to the evaporation direction was 2.9.