In the past decades organic thin film transistors (OTFTs) have been notably studied due to their interesting properties. Not only they can be processed by simple methods such as inkjet printing but also open the doors to new applications for cheap plastic electronics including electronic tags, biosensors, flexible screens,… However, the measured field-effect mobility in OTFTs is relatively low compared to inorganic devices. Generally, such low field-effect mobility values result from extrinsic effects such as grain boundaries or imperfect interfaces with source and drain electrodes. It has been shown that reducing the number of grain boundaries between the source and drain electrodes improves the field effect mobility.1-3 Therefore, it is important to understand the transport mechanisms by studying the structure of organic thin films and local electrical properties within the channel and at the interfaces with source and drain electrodes in order to improve the field-effect mobility in OTFTs. Kelvin probe force microscopy (KPFM) is an ideal tool for that purpose since it allows to simultaneously investigation of the local structure and the electrical potential distribution in electronic devices. In this work, the structure and the electrical properties of OTFTs based on dioctylterthiophene (DOTT) were studied. The transistors were fabricated by spin-coating of DOTT on the transistor structures with treated (silanized) and untreated channel oxide. The potential profiles across the channel and at the metal-electrode interfaces were measured by KPFM. The effect of surface treatment on hysteresis effects was also studied. Smaller crystals and a lower threshold voltage were observed for the silanized devices. Hysteresis effects appeared to be less important in modified devices compared to the untreated ones.