To prepare nanosized graphene-like molecules of a defined structure (defined-width graphene nanoribbons or nanofragments) by a simple bottom-up method, thermal polymerization reactions of pentacenes were investigated. By optimizing heat treatment temperature and initial precursor weight, long-length fused pentacene molecules were successfully obtained at least up to octamer (n = 8). Here, the degree of polymerization was much larger than that of previously known polymerized pentacene systems (n = 2, 3). The structural and physical properties of the obtained fused pentacenes were characterized by Raman spectroscopy, X-ray diffraction, and photoluminescent spectroscopy. The fused pentacene system, examined using density functional theory calculations, was found to have unique electronic and magnetic structures originating from its characteristic size and edge structure. In addition, we performed detailed mass spectroscopic analysis that examined the fusing mechanism.