Atomic force microscopy was used to investigate the mixed monolayer of FC- and HC-amphiphiles. Clear topographic images showing domain structure of the mixed monolayer and the data of the domain size have been provided. It is revealed that in the mixed monolayer the sodium octadecanesulfonate molecules construct the hexagonal domain dispersing in the continuous two-dimensional phase of perfluorononanoic acid. These results strongly support that, due to the mutual-phobicity between fluorocarbon chain and hydrocarbon chain, there exists phase separation in the mixed monolayer of fluorocarbon and hydrocarbon amphiphiles, albeit the pi-A curves would obey the simple additive rule.