The thermochromic phase-transition behavior of polydiacetylene (PDA) Langmuir-Blodgett (LB) films in the blue form and the bluish-green one was investigated from the viewpoint of the molecular lateral packings evaluated by the electron diffraction (ED) analysis. The PDA blue and bluish-green forms exhibit absorption peaks at the wavelengths 640 and 704 nm in the visible absorption spectrum, respectively At an elevated temperature, the PDA LB films in both forms were converted into the red form with the absorption peak at the wavelength 540 nm. The chromatic phase transition of the PDA LB films was accompanied by the characteristic crystal phase transition in both cases. Further, the detailed structural analyses based on the ED measurement, the Fourier transform infrared external reflection spectrum, small-angle X-ray scattering, and X-ray photoelectron spectrum measurements revealed that the difference in the chromatic phase of the PDA LB film could be explained by the difference in the exciton energy level being strongly dependent on both the crystal symmetry and the resonance backbone structure.