The aggregation state of a mixed monolayer of cyanine dye and fatty acid during compression was investigated on the basis of surface pressure-area isotherm measurements, atomic force microscopic observations and spectroscopic measurements. The mixed monolayer was in a miscible state at all surface pressure values. The domains were formed in a matrix region by compression, which is probably due to ordering of molecular conformation. Compression at a surface pressure higher than the plateau pressure causes regularization of stacking of cyanine rings, resulting in the formation of J-aggregates.