Organic ultrathin transistors with a 3-μm-length channel were fabricated using dimethylquinquethiophene molecules. The local structures of the films and electric potential profiles across the channels were investigated by Kelvin probe force microscopy with the gate electrode grounded. We found that the local carrier mobility was much higher for the second-layer regions than for the monolayer regions from potential profiles measured while the gate electrode was electrically grounded. However, the macroscopic carrier mobility increased only slightly by increasing the number of molecular layers. One of the possible reasons for this contradiction was the large contact resistance at the source/channel interface, which was apparently observed in the potential profiles.