To improve the performance of organic field-effect transistors (OFETs) composed of poly(3-hexylthiophene) (P3HT) semiconductor and polymethylsilsesquioxane (PMSQ)-based insulator, Surface modification of PMSQ insulator has been carried out. The modified PMSQ were synthesized by a sol-gel co-condensation of methyltrimethoxysilane and hydrophobic alkoxysilane. The thin films from these PMSQ had smooth and hydrophobic surface. Resulting OFET fabricated on the modified PMSQ insulator showed enhanced mobility. A clear relation Could be seen between the mobility and the Surface free energy of insulator.