The surface states of gate dielectrics affect device performance severely in Pentacene OTFTs. We have fabricated organic thin-film transistors (OTFTs) using pentacene as an active layer with chemically modified $SiO_2$ gate dielectrics. The effects of the surface treatment of $SiO_2$ on the electric characteristics of OTFTS were investigated. The surface of $SiO_2$ gate dielectric was treated by normal wet cleaning process, $O_2-plasma$ treatment, hexamethyldisilazane (HMDS), and octadecyltrichlorosilane (OTS) treatment. After the surface treatments, the contact angles and surface free energies were measured in order to analyze the surface state changes. In the electrical measurements, typical I-V characteristics of TFTs were observed. The field effect mobility, $mu$, was calculated to be $0.29;cm^2V^{-1}s^{-1}$ for OTS treated sample while those for the HMDS, $O_2$ plasma treated, and wet-cleaned samples were 0.16, 0.1, and $0.04;cm^2V^{-1}s^{-1}$, respectively.