This report is the first systematic study of the temperature effect on polymerization of lithium 10,12-heptacosadiynoate monolayer with relation to its aggregation structure. A molecular aggregation state of the monolayer on the water surface was investigated on the basis of the subphase temperature (T-sp) dependences of the elastic modulus and the electron diffraction (ED) pattern of the monolayer. The monolayer on the water surface was classified into a molten monolayer, a crystalline one and a glassy one, depending on T-sp in comparison with the melting temperature (T-m) of the monolayer on the water surface. The photopolymerization behaviors of the monolayers in various aggregation states were also investigated by the ultraviolet (UV) light irradiation time dependences of UV-visible absorption spectrum. The photopolymerization was less reactive in the case of the monolayer in a molten state. On the other hand, the poly(diacetylene) (PDA) blue form monolayer, which had the absorption peak at 640 nm, was formed upon photoirradiation to the crystalline monolayer. Moreover, in the case of the glassy monolayer, PDA red form monolayer, which had the absorption peak at 540 nm, was found to be formed by UV light irradiation. The delocalization length of a pi-electron in the PDA red form would be shorter than that in the PDA blue form, as suggested by the wavelength of the main absorption peak corresponding to pi-pi* transition. The difference in the delocalization length of pi-electron between the PDA blue form and the PDA red form could be explained toy the lattice strain on conjugated PDA main chains caused during the polymerization reaction.