A naphthalene diimide (NDI)-based small molecule with a thienothiophene linker (NDI-TT-NDI) was synthesized for use as a semiconductor layer in solution-processable n-type organic field-effect transistors (OFETs). The thienothiophene linker was introduced to the donor site between the two NDIs to impart planarity to the molecular backbone, which is favorable for inducing high crystallinity, and the morphological and crystal characteristics of the resulting film were analyzed. An additional annealing treatment applied to the NDT-TT-NDI films changed their orientation such that the molecular ordering and crystallinity of the film were significantly improved at a specific annealing temperature. The optimally annealed NDI-TT-NDI film exhibited a distinct edge-on molecular orientation with a narrow intermolecular π−π stacking distance, which is advantageous for lateral charge transport along the stacks. Consequently, an optimally annealed NDI-TT-NDI-based OFET exhibited electron mobilities of up to 0.032 cm2V−1s−1 and an on/off ratio of 1.0 × 107.