In this paper, the electrical behavior of a quartz crystal microbalance (QCM) with dye LB film in UV irradiation has been examined by the impedance analysis method. The equivalent electrical parameters, such as resonant frequency (f(0)) and resistance (R-1), were investigated. The preparation conditions, using a mixing ratio of fatty acids, subphase effect, and Tr-A isotherms were investigated to obtain the J-aggregated dye LB film and its optical characteristics were evaluated by UV spectroscopy. The resonant frequency was originated without considering mass adsorption and dependence on the mixing ratio with arachidic acid or resultant absorbance peak at the J-band (optical characteristics). This result could be described by the observation of morphological change before and after UV irradiation, i.e. the resonant frequency shift had a connection with the morphological change. This result strongly suggests that the QCM can be applied to monitor the structural change or optical characteristics of an organic film on the QCM.