To modify SiO2 dielectric surface of bottom-contact OTFT device, octadecyltriethoxysilane (OTES) was used to find optimum condition of surface modification for triethoxysilane derivatives. Either spin-coating method or solution dipping method was applied to modify the dielectric surface with OTES. Optimization process was performed with varying solution concentration, reaction time and so on. Through surface modification of OTFT 0.01-0.04 cm(2)/V/s of hole mobilities were observed depending on modification conditions.