Solid-state polymerization behaviors of crystalline lithium 10, 12-heptacosadiynoate monolayers on the hydrophilic SiO substrate and on the water surface were investigated on the basis of the ultraviolet (UV) light irradiation time dependence of the UV-visible absorption spectrum. In the case of the crystalline monolayer on the SiO substrate, the polydiacetylene (PDA) monolayer in a blue form was formed upon UV photoirradiation. On the other hand, in the case of the crystalline monolayer on the water surface, the PDA monolayer in a red form was formed by the polymerization reaction. The PDA blue and red forms exhibit absorption peaks at 640 and 540 nm, respectively. The two-dimensional (2D) molecular packings of the PDA monolayers in both forms were clarified by the electron diffraction (ED) analysis for the first time. Each form had the distinct 2D molecular packing relating to its electronic structure. The difference in the solid-state polymerization behaviors for the crystalline lithium 10, 12-heptacosadiynoate monolayers on the different substrate surfaces was discussed in terms of the thermal mobility of molecules in the monolayer.