We fabricated pentacene OTFTs with chemically modified SiO2 surface by phenyltrichlorosilane (PTS), and studied the molecular ordering of the pentacene films and electrical characteristics of the OTFTs. The molecular ordering of the pentacene thin film was improved by PTS treatment. We also fabricated organic thin film transistors with phenyltrichlorosilane treatment. The carrier mobility of the transistor with PTS treatment was 3.7 x 10(-2)cm(2)/Vs, it was larger than that of the transistor without PTS treatment (2.0 x 10(-2)cm(2)/Vs). We believe that this improvement of the value was due to improvement of the molecular ordering by PTS treatment.