The electrical characteristics of organic thin film transistors (OTFTs) with self-assembled monolayer (SAM) on the poly(4-vinylphenol) gate insulator have been investigated. The SAM was formed in gas-phase using the atomic layer deposition method onto gate insulator. It is observed that SAM modifies the surface polarity of gate insulator and influence the growth of the subsequent pentacene layer. Improvements of the crystallinity and directionality of pentacene molecules are confirmed by X-ray diffraction measurement. Also, the roughness of the pentacene thin film and the device performance are improved.