Asymmetrically-substituted smectic liquid crystals with a small core of benzo[1,2-b:4,5-b'] dithiophene (BDT) substituted with monoalkyl and phenyl groups, 6-alkyl-2-phenyl-benzo[1,2b:4,5-b']dithiophene (Ph-BDT-Cn, n = 8, 14), were synthesized, and their mesomorphism and properties of crystalline films were characterized for solution-processed OFETs. Both compounds exhibited the SmE phase, and their crystalline films derived from SmE films exhibited the crystal-to-crystal transition from the monolayered crystals as-fabricated to the bilayered crystals when thermally annealed. The FET mobility was significantly enhanced by a factor of 2 to 10, accompanied with the phase transition to the bilayered crystals and reached up to 0.59 cm(2) V-1 s (-1) in spite of a small core of BDT. We discuss a new strategy of molecular design of soluble OFET materials behind the present results.