The copolymer of polythiophene derivative (P3HT-co-P3AcET) containing alkyl and ester side chain was prepared by FeCl₃ oxidative coupling copolymerization. The chemical structure of the resulted copolymer was characterized by GPC, ¹H NMR, et al. The assembling of P3HT-co-P3AcET nanostructures was performed by mixing solvents, of which toluene was adopted as the good solvent, while dioxane or methanol as the poor solvent. The nanostructures of P3HT-co-P3AcET with different morphologies were obtained by adjusting the ratio of the mixed solvents and the aging time. The SEM results show that the assembling of P3HT-co-P3AcET tends to produce nanofiber bundles with dioxane as the poor solvent, and spherical nanoparticles with methanol as the poor solvent in the mixed solvents, respectively. For which a higher lipophilic value of dioxane rather than that of methanol may be responsible. The arrangement manner of the copolymer chains was analyzed by ultraviolet visible light absorption spectrum. The results show that the different arrangement manners of the copolymer chains can give rise to different morphologies in nanoscale, such as nanofiber bundles and spherical nanoparticles. Finally the mechanism concerning the formed nanostructures of the assembled copolymer was also preliminarily explored.