In the mixed Langmuir-Blodgett (LB) films of a selenium-containing cyanine dye and an azobenzene derivative (molecular ratio = 3:1), the azobenzene photoisomerized reversibly by the alternate illumination with UV and visible light. After three cycles of photoisomerization, the J-band of the cyanine dye started to develop and reached a saturated state after 12 h, when the samples were stored in the dark. A drastic change in the morphology of the mixed LB films occurred with the development of the J-band. When the J-band was fully developed, a number of cones protruded from the film surface which should be due to the formation of J-aggregate of the cyanine dye. The size of the largest cones was ca. 250 nm in height and ca. 2000 nm in diameter at the base. The light-induced J-aggregation in the present case is similar in nature to the previously reported case (M. Matsumoto et al., J. Phys. Chem. B 101 (1997) 702). However, the number of cones is smaller, the size larger, and the time necessary for the full development of the cones longer in the present case compared with the previous case.