Ab initio crystal orbital (CO) studies on the geometric and the electronic structures of poly(thienothiadiazole) (Poly-1a) and periodic copolymers of thienothiadiazole and thiophene with the ratio of 1:1 (Poly-2b) and 1:2 (Poly-3b) are presented. In Poly-1a and Poly-2b, considerable change in the geometries is found to occur as one moves from their oligomers to the polymers. Although thienothiadiazole oligomers have a very small highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) separation, the corresponding polymer (Poly-1a) has a considerably large band gap. On the contrary, the geometric and electronic structures of Poly-3b are almost identical to those of the oligomer, and Poly-3b possesses a direct band gap of 1.3 eV estimated by simple scaling. The reasons for these differences are discussed in terms of orbital interactions and nonbonding molecular orbitals in the oligomers, and the reduced pi-conjugation and enhanced intercell interaction in the polymers.