Formation of J-aggregates of a cyanide dye (S120) and its two-dimensional crystalline domain structures at the air/water interface was examined with fluorescence microscopy. A new preparation technique of a 'simultaneous spreading-compression method' (SSC method) was successfully developed in order to fabricate LB films which have domains of larger size than films prepared by the conventional method. The effects of solvent and spreading-compression procedures on the behavior of the surface pressure of the monolayer and on the formation of the J-aggregates and their domain structures were discussed.