We report on the room-temperature self-organizing characteristics of thin films of the organic small-molecule semiconductor triethylsilylethynyl-anthradithiophene (TES-ADT) and its effect on the electrical properties of TES-ADT-based field-effect transistors (FETs). The morphology of TES-ADT films changed dramatically with time, and the field-effect mobility of FETs based on these films increased about 100-fold after seven days as a result of the change in molecular orientation from a tilted structure in the as-prepared film to a well-oriented structure in the final film. We found that the molecular movement is large enough to induce a conformational. change to an energetically stable state in spin-coated TES-ADT films, because TES-ADT has a low glass-transition temperature (around room temperature), Our findings demonstrate that organic small-molecule semiconductors that exhibit a low crystallinity immediately after spin coating can be changed into highly crystalline structures by spontaneous self-organization of the molecules at room temperature, which results in improved electrical properties of FETs based on these semiconductors.