Polymer-based thin film transistors (PTFTs) with poly(3-hexylthiophene) (P3HT) as semiconducting active layers are successfully fabricated on silicon substrates in air which are used as gate electrods. HfTiO film deposited by RF sputtering method is used as gate insulators,and gold metal is used as source and drain electrodes. The HfTiO surface is modified by using octadecyltrichlorosilane solution in the fabrication process. Experimental results indicate that the PTFTs show good saturation behaviors at low drive voltages (-2 cm2/V ·s. The frequency-dependence and hysteresis effect are observed in the C-V measurements for the metal-polymer-oxide-silicon (MPOS) capacitors,and discussed in detail to understand their physical mechanism.