Epitaxial structuring of blue light-emitting molecules, p-phenylene oligomers (p-nP, n = 3,4,5,6), was investigated in thin crystals vapor deposited on the (001) surface of a KCl single crystal. Their molecular orientation and morphology changed depending on the molecular length and substrate temperatures during deposition. As the length of p-nP was longer and the substrate temperature was lower, the orientation of the molecular axis changed from standing to lying on the KCl surface due to an increase of molecule–substrate interactions. p-terphenyl (3P) hardly adsorbed on the KCl (001) surface, but a portion of the standing molecules stuck along the step edge of KCl and formed a walllike structure. Long needlelike crystals of p-hexaphenyl (6P) epitaxially grew in two orthogonal KCl  directions taking lying orientation even at a higher substrate temperature. In these epitaxial needles, the transition dipoles parallel to the molecular axis gave rise to blue fluorescence sharply polarized along the KCl  directions. © 1998 American Institute of Physics.