An amphiphilic diacetylene with thymine (DA-Thy) was newly synthesized. The amphiphile can form solid-state monolayer and photopolymerize with UV light irradiation at the air-water interface to form blue and red forms of polydiacetylene. Shapes of the crystalline domain of the polydiacetylene monolayer were changed by base pair formation between DA-Thy and adenine nucleobase dissolved in the water subphase.