Relationship between molecular structure and electrical performance of oligothiophene organic thin film transistor (TFT) was studied. The performance of TFT's with ultra-thin self-assembled octadecyltrichlorosilane (OTS) monolayer gate dielectrics deposited by evaporation was found to be more sensitive to the choice of semiconductors. The long side chains lead to an increase in the ethyl-substituted sexithiophene in the effective thickness of the gate dielectric, with reduced gate currents and current mobilities.