Crystallization of P3HT on highly oriented PE ultrathin films via solution-deposition has been studied by means of optical microscopy, atomic force microscopy, Fourier transform infrared spectroscopy (FTIR), and electron diffraction. The results clearly indicated the occurrence of hetero-epitaxy of P3HT on the PE substrate, which results in a parallel alignment of P3HT on the PE substrate. FTIR spectra and electron diffraction analyses demonstrate that molecular chains of P3HT are oriented in the film plane and aligned parallel to the chain direction of PE substrate crystals, while the (100) lattice plane of P3HT is in contact with the PE substrate. The observed epitaxy can be explained in terms of an excellent one-dimensional lattice matching between the interchain distances of PE in the (110) lattice plane and P3HT in the (100) lattice plane. This kind of epitaxial crystallization provides an efficient way for fabricating large area P3HT films with unique oriented structures.