This study examined the electrical properties of 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES-ADT)/poly(methyl methacrylate) (PMMA) blend semiconductors for fabricating high-performance organic field-effect transistors (OFETs). The phase-separation characteristics were investigated over a range of blend solution concentrations. Regardless of the concentration, diF-TES-ADT crystal-top/PMMA-bottom bilayer structures were formed onto SiO2/Si substrates owing to the surface energy difference between diF-TES-ADT and PMMA. The phase-separated interfaces between diF-TES-ADT and PMMA provided efficient pathways for charge transport. Consequently, a high field-effect mobility of 0.1 cm2/V s and a current on/off ratio of 107 were achieved at the optimum concentration of 20 mg/ml where thickness of phase-separated diF-TES-ADT crystal is 20 nm. When the concentration was too low (e.g., 10 mg/ml) or too high (e.g., 30, 40, and 50 mg/ml), field-effect mobility was reduced significantly. It can be concluded that this is due to the homogeneity and thickness of the phase-separated diF-TES-ADT crystals.