A bolaamphiphilic compound containing photoactive naphthyl group, 1,20-bi(α-naphthol)octadecane-dicarboxylate (C20BNp) was newly synthesized. The spreading property of the compound at the air/water interface was investigated by the surface pressure (π).area (A) isotherm and in situ Brewster angle microscopy (BAM). It has been found that C20BNp could self-aggregate into a two-dimensional (2D) crystalline film on water surface. Upon irradiation (365 nm) in N2 atmosphere, photochemical reaction in the Langmuir film occurred as detected from the surface pressure-time measurement and in situ BAM observation. The 2D crystal film on water surface could be deposited onto solid substrates by vertical dipping to form a wellorganized Langmuir-Blodgett (LB) film, as verified from the X-ray diffraction measurement. Upon irradiation, photochemical reaction could also occur in the LB film as verified from the ultraviolet and fluorescence measurement. While the atomic force microscopy image of the LB film appeared as a flat rectangle crystal, many holes have been observed after photoirradiation.