Langmuir-Blodgett (LB) films containing functional dyes were deposited directly onto optical fibers by the adsorption LB method. A merocyanine derivative having a long alkyl chain and rhodamine B were used as spreading and adsorbing molecules from the aqueous subphase, respectively. During the LB film deposition, absorption spectra from the optical interaction with the LB films at the core portion of the fiber were monitored using a multi-channel photo-detector system. Furthermore, the influence of the excitation light, which was guided into the fiber during the deposition, on the film deposition was investigated. The experimental results indicated that the in-situ optical measurement using optical fibers has high sensitivity and this is one of the promising methods to investigate the molecular interactions and the effect of the incident light on the LB film deposition.