An amphiphilic diacetylene, 10,12-pentacosadiynoic acid, was in-situ topochemically polymerized within a TiO2 nanostructure under visible light irradiation. The red shift of the wavelength at which the polymerization occurs can be attributed to the photocatalytic effect of TiO2 upon absorption of irradiation. Since visible light can be transmitted through the entire nanoporous TiO2 film, it is possible to obtain complete polymerization of the diacetylene monomers inside the TiO2 nanopores so as to provide an approach toward fabrication of the polydiacetylene/TiO2 nanocomposite structure and further development of optoelectronic devices.