Thin film of J-aggregate of merocyanine (MD-18) was prepared by exposure the vacuum-deposited MD-18 film to dimethyl amine (DMA) aqueous vapor. The structure of J-aggregate of MD-18 was investigated by X-ray photoelectron spectroscopy (XPS). The intensity of N1s and O1s spectra increased by DMAaqueous vapor treatment. J-aggregate of MD-18 contained hydrated DMA. MD-18 molecules were combined each other with hydrogen bonding through hydrated DMA.