Three low bandgap copolymers of thiophene and benzothiadiazole with electron-donating and electron-withdrawing substituents, P1, P2, and P3, have been synthesized by Pd-catalyzed Stille-coupling. Electronic energy levels of the polymers are estimated by cyclic voltammetry. The polymer films show a broad absorption band in the wavelength range from 300 to 750 nm. can effectively decrease the bandgap of the polymers. Among Me polymers, the polymer that contains the 5,6-dinitrobenzothiadiazole unit, P3, possesses the smallest bandgap of 1.55 eV calculated from its absorption band-edge at approximate to 800 nm. With the increase of the electron-withdrawing ability of the substituents on the benzothiadiazole unit, the energy bandgap of the polymers decreased in the order P1 > P2 > P3. The results indicate that stronger electron-withdrawing substituents on the acceptor unit