Multi-walled carbon nanotubes (MWNTs) were used as filler to prepare polythiophene (PTh)/MWNTs composites, which have a high conducting performance (16.1 S/m). Raman spectra, thermogravimetric (TG), X-ray spectroscopy (XPS) and ultraviolet-visible absorption spectra (UV-Vis) were employed to characterize these composites. The results suggest that an intensive interaction between MWNTs and PTh does exist. A π-π system is formed between delocalized electrons in MWNTs and the π-electrons in the conjugated chain of PTh. The electrons transfer from MWNTs to PTh through this π-π system, which can prolong the effective conjugated chain of PTh and improve the conductibility of the composites. Field emission scanning electron microscopy (FESEM) analysis indicates that MWNTs are combined with the doped PTh to form individual conductive units, which play a role in the conducting system of the composites.