A series of donor-acceptor oligomer OBTThn (n=1–7) and polymer PBTTh1 and PBTTh2 composed of alternative 2,1,3-benzothiadiazole and 3-hexylthiophene have been designed and synthesized for the purpose of investigation on the effect of chain length and side-chain regioregularity on their basic properties and photovoltaic performance. In the OBTThn oligomers and PBTTh1 polymer, all the hexyl side chains on thienyl units orient toward the same direction. Upon elongation of the chain length, the intramolecular charge transfer (ICT) absorption band in solution gradually redshifts from 398 nm for OBTTh1 to 505 nm for OBTTh7, then to 512 nm for PBTTh1 polymer. Meanwhile, the HOMO energy level increases from −5.45 eV (OBTTh1) to −5.08 eV (OBTTh7) and −5.09 eV (PBTTh1), and the LUMO energy level decreases from −3.11 eV (OBTTh1) to −3.30 eV (OBTTh7) and −3.33 eV (PBTTh1), thus giving a smaller and smaller energy bandgap for higher oligomers and polymers. Theoretical calculation suggests straight line-like backbone geometry for this series of oligomers and polymer. On the other hand, polymer PBTTh2 possesses a different side-chain regioregularity, in which every two neighbor hexyl side chains are arranged in different orienting direction. It is theoretically suggested to have curved line-like backbone geometry. In solution, it shows similar photophysical and electrochemical properties as PBTTh1. However in film state, it displays a less redshift in the ICT band as refer to that in solution than PBTTh1. In combination with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), these oligomers and polymers were used as donor material to fabricate organic bulk heterojunction solar cells. Again, chain length-dependent device photovoltaic performance was observed. The device based on OBTTh4 showed a power conversion efficiency of 0.16%, while it increased to 0.36% and 0.49% for the devices based on OBTTh6 and PBTTh1, respectively. However, the side-chain regioregularity has less influence on the device photovoltaic output since the device based on PBTTh2 displayed an efficiency of 0.52%, comparable to that of PBTTh1.