Photoinduced cycloaddition of a p-phenylenediacrylic acid derivative, 4-(4-(2-(decyloxycarbonyl)vinyl)cinnamoylamino)benzoic acid, was conducted successfully in a mixed Langmuir monolayer with stearic-d(35) acid on a water surface at room temperature. The reaction process was investigated by W-visible and infrared spectroscopy, gel permeation chromatography (GPC), and Brewster-angle microscopy (BAM). The product was identified as the cyclobutane dimer. The formation of the stable monolayer and the dimerization reaction on the water surface were evaluated at the molecular level by observation of the monolayer＇s behavior (surface pressure, monolayer area, and change of surface morphology). The BAM observation and GPC analysis of the photoproducts obtained under different states of the monolayer reveal that the molecular arrangement with higher mobility gives a much higher reaction speed.