This research focused on the effect of molecular weight and investigated the conformational characteristics of poly(3-hexyl thiophene) (P3HT). An integrated system consisting of a gel permeation chromatograph, a static light scattering unit, and a viscometer was used for the study. It also estimated the radius of gyration (R g ) values of unsubstituted poly(thiophene) (PT) chains computationally using rotational-isomericstates modeling and compared them with the experimental data for P3HT. In the low molecular weight region (20,000 to 30,000), both chains had nearly the same R g values, meaning that the effect of side-chains is limited. At higher molecular weights, the P3HT chains expanded more than the PT chains. In the molecular weight region from 20,000 to 60,000, both characteristic ratio and persistence length showed considerable molecular weight dependence. Beyond a molecular weight of 60,000, the molecular weight dependence decreased, and these parameters approached constant values, 16 and 3 nm, respectively.