Polythiophene (PTh)/multi-walled carbon nanotube (MWNT) composites were investigated on their structure and the interaction between PTh and MWNT. On one hand, the structure of PTh has an effective influence on the conductive properties of the composites. On the other hand, PTh is doped by MWNT. An intense interaction exists between MWNT and PTh when the electrons transfer from MWNT to PTh. MWNT can combine with the doped PTh via π-π system to form individual conductive units which play a critical role in the conducting system. With the increase of the conductive units, a larger conducting system is formed by conductive unit connection and the conductivity of composites reaches the maximum.