A single crystal polyaniline membrane was deposited on a crystalline HDPE (high density polyethylene) via an in-situ polymerization/deposition method. Both crystalline and amorphous polymers were used as substrates to prove that the deposition process is an epitaxial growth. Single crystal polyaniline membranes have not only high conductivity but also good conducting and doping homogeneity. The epitaxial growth of the conducting polymer on the flexible polymeric substrate is reported for the first time and the mechanism for the epitaxial growth of polyaniline chains on HDPE was proposed. Being able to epitaxially grow highly crystalline and good conducting homogeneity conjugated polymer membranes on flexible polymeric substrates provides a very promising step towards preparing high conducting polymer films or high mobility polymer dielectrics for flexible organic electronic applications.