<p>The electrical characteristics of organic transistors are generally suffered from the effects of interfacial resistance (<i>R</i>_<i>i</i>) between source/drain electrodes and organic semiconducting layer, which leads to a nonlinear increase of current at low drain voltage region. In this paper, the <i>R</i>_<i>i</i> value is first estimated by transfer line method from the devices with various channel lengths (range from 5 to 100&#xa0;μm) at a constant channel width (500&#xa0;μm). The extracted interfacial resistance exhibits a remarkable dependence on gate voltage attributed to the reduction of contact barrier depicting by classic charge-carrier’s drift–diffusion mode for metal/semiconductor contact. The two field-effect mobilities are extracted from linear and saturation region, which are suffered from the <i>R</i>_<i>i</i> effects, and in the linear region the effect is more significant. And then we corrected the field-effect mobility by deducting the drain voltage drop (<i>I</i>_<i>D</i><i>R</i>_<i>i</i>) brought by <i>R</i>_<i>i</i>. The corrected mobility is approximate constant and independent of channel length, which may be used to well evaluate the intrinsic charge-transport capability of organic semiconductor. Finally, we present the impacts of relevant parameters including metal electrodes, thin film morphology and interface order on the interfacial resistance. The organic thin film with larger grain size brings the superior contact characteristics due to its high continuity and order at the interface of metal/insulator. Current studies will be helpful for in-depth understanding the function of contact properties in OFET.</p>