A water‐soluble cyanine dye is adsorbed from the solution to form a salt with an arachidic acid monolayer at the air‐water interface. Complex monolayers thus formed were deposited onto substrates by the vertical dipping method to form Langmuir–Blodgett (LB) films. The optical properties of these films were examined employing a linearly polarized incident light. The films showed a large in‐plane anisotropy of light absorption in both visible (electronic transition) and infrared (vibrational transition) regions. The in‐plane anisotropy of the LB film is explained as due to the alignment of dimers, each showing Davydov splitting, by flow orientation during dipping. The configuration of the dye molecules in the dimer was estimated using Davydov’s theory and the extended dipole model. The layered structure of the film was determined by the x‐ray diffraction technique.