Organic thin film transistors (OTFTs) have drawn much attention for flexible electronics such as smart cards, flexible displays, and several types of low cost and low-end electronics. Recently, OTFTs with field-effect mobility and on/off ratio current ratio comparable to a-Si:H thin film transistors have been fabricated. Since the electrical conduction of OTFTs is highly affected by the molecular ordering, crystalline orientation, and the crystallized grain size of the organic semiconductors, there were several efforts to modify interfacial properties between the evaporated pentacene molecules and the insulator surface. In general, highly oriented polymeric layers by mechanical rubbing or irradiation of polarized UV light were used for the purpose. But, the electrical performances of OTFTs on the organic insulators are not satisfactory yet, comparing with those on the inorganic insulators, especially in terms of leakage current, the driving gate voltage, and stability in ambient condition. In this paper, we investigated anisotropic conduction effects of pentacene-based OTFT on an anisotropic insulator fabricated by obliquely evaporated silicon dioxide. As the evaporation angle of SiO₂ increased, the anisotropic interaction at the dielectric interface and molecular ordering of evaporated pentacenes increased. However, in highly obliquely evaporated dielectric surface, it was observed that growth of the pentacene molecules was highly limited due to increased roughness at the inteface. The molecular ordering effect and the grain size effect depending on the surface anisotropy and the surface roughness were discussed with the results of the field-induced mobility and the surface morphology. We believe that further optimization of oblique evaporation in OTFTs would result in more anisotropic electrical property.