In this work, a room-temperature, printed, low-voltage, flexible organic field-effect transistor (OFET) has been successfully developed by utilizing 4,4′-(hexafluoroisopropylidene)diphthalic anhydride-3,5-diaminobenzyl cinnamate (6FDA-DABC) and diketopyrrolopyrrole-dithienylthieno[3,2-b]thiophene (DPP-DTT) as polymer insulator and semiconductor layers, respectively. Dielectric properties are systematically evaluated to investigate the room-temperature processability of 6FDA-DABC. In addition, the introduction of insulating polymer, polystyrene (PS), blends considerably improves the electrical characteristics of DPP-DTT-based OFETs. The operation voltage is successfully lowered to −5 V by reducing the gate dielectric thickness. OFETs based on DPP-DTT:PS annealed under various temperature conditions demonstrate the fully room-temperature processability. Finally, OFETs integrated with ultrathin flexible substrates exhibit excellent mechanical flexibility while maintaining device performance. This work provides a great freedom in the choice of plastic substrates for the development of flexible electronic applications.