The formation processes of polydiacetylene (PDA) Langmuir-Blodgett (LB) films in the blue and the bluish-green forms were investigated from the viewpoint of the molecular lateral packings evaluated by electron diffraction (ED) analysis, The PDA blue and bluish-green forms exhibit absorption peaks at 640 and 704 nm in the visible absorption spectrum, respectively. During the formation process of the PDA blue form, the two-dimensional unit cell of the monomeric LB film shrunk due to the covalent band formation among the molecules. On the other hand, during the formation process of the PDA bluish-green form, there was no change in the molecular packings. Also, the ED analysis revealed the difference in tile repeating unit distances of main chains between both forms, implying that both forms had distinct resonance backbone structures. It was concluded from comparison of the ED analysis and tile molecular mechanics calculation that the bluish-green ferns was composed of the butatrienic type main chains rather than the acetylenic ones corresponding to the resonance backbone structure of the blue form.