In this study, we investigated the optical characteristics of merocyanine dye Langmuir-Blodgett (LB) film and had analysis combined with surface morphology using AFM. And preparation condition for merocyanine dye LB film was determined. As a result, domains could be observed in the J-aggregate LB film and domain size was dependent on the mixing ratio. And the domain had very homogeneous two-dimensional structure. After UV irradiation, the domain size was decreased or disappeared correspond to the decrease of absorbance peak, which mean the dissociation of J-aggregate. These results can suggest the modulation of structure by light.