A new terthiophene monomer: 3',4'-dimethoxy-2,2':5',2"-terthiophene (TMT) was synthesized and characterized by ¹H-NMR, ¹³C-NMR and FTIR. The solid-state oxidative polymerizations of TMT were performed in various ratios of oxidant (FeCl₃) to monomer (TMT). The resulting polymers were characterized by ¹H-NMR, FTIR, UV-vis-NIR, GPC, X-ray diffraction, CV, as well as TGA and conductivity measurements. The structure and properties of poly (TMT) were compared with those of polyterthiophene [poly(TT)] and poly (3',4'-ethylenedioxy-2,2':5',2"-terthiophene) [poly(TET)] prepared under the same polymerization conditions. After comparative analysis with poly(TT) and poly(TET), the effects of the dimethoxy substituent and FeCl₃ on the structural and physicochemical properties of the poly(TMT)s were discussed in depth. The comparison suggested that the dimethoxy-substituted polymer did not display higher crystallinity, thermal stability, conductivity and electrochemical activity than ethylenedioxy substituted one. The results also showed that the effect of FeCl₃ on poly(TMT) was similar that seen with the poly(TT), in which the oxidation degree, electrochemical activity and conductivity increased steadily with increasing [FeCl₃/[TT] ratio. Furthermore, the poly(TMT) and poly(TT) are mostly made up of dimers with a small amount of higher molecular weight components.