A characteristic microdomain structure in monolayers of the cadmium salt of heptadecafluorononadecanoic acid was found by atomic force microscopy. The density of the microdomain increases with increasing surface pressure, and we can observe a micronodule structure above 20 mN m(-1). The diameter of the micronodule was 17 nm on average and the height difference of the top and the bottom of the micronodule was about 1 nm. The formation mechanism of such a characteristic structure is proposed and discussed.