Herein, a series of asymmetric monoalkyl terthiophene derivatives, 5-phenyl-5''-alkyl-2,2':5',2''-terthiophene (Ph-(Tp)3-Cn, 6 ≤ n ≤ 18, where n represents the number of carbons in the alkyl group), were synthesized to investigate the formation of monolayered crystals derived from the smectic phase and their phase transition to bilayered crystals, leading to a significant increase of FET mobility when applied to organic field-effect transistors. All compounds showed a highly ordered smectic phase, i.e., SmE phase, and gave a monolayered crystal when spin-coated at SmE temperature. It was found that phase transition from monolayered to bilayered crystals was induced by thermal annealing when the alkyl side chain length n ≥ 10, and OFET mobility in FETs fabricated with polycrystalline thin films was dramatically enhanced up to one order of magnitude when the bilayered crystals were formed. This result proves that this phase transition is not limited to a particular type of smectic liquid crystals consisting of an aromatic fused ring system reported in benzothienobenzothiphene (BTBT) as a core derivative and can be extended to a variety of core systems, e.g., oligomer type of smectic liquid crystals. These findings may provide a new strategy for molecular design to boost mobility in soluble OFET materials.