The electrophoretic deposition of nanostructured films from colloidal suspensions of a conjugated polymer poly(3-octadecylthiophene) (PAT18), as well as the preparation of the suspension by a simple mixing method, is reported. A colloidal suspension of PAT18 was prepared by pouring a toluene solution of the polymer into acetonitrile, which is not a solvent for the polymer. It was found that the suspension obtained was stable for more than two weeks, and the stability was related to the concentration of the polymer therein. In comparison with a toluene solution of PAT18, the colloidal suspension shows a considerable red-shift in the optical absorption and photoluminescence spectra, as well as a quenched photoluminescence, indicating the solidification of PAT18 in the suspension. The electrophoretic deposition of a PAT18 film from the colloidal suspension was successfully carried out. The film shows almost the same optical absorption and photoluminescence spectra as those of a spin-coated film. On the other hand, an atomic force microscopy study revealed a nanostructured surface morphology of the electrophoretically deposited films. Such nanostructured films are expected to be promising materials for electrochemical and sensor applications.