The electric repulsive energy of circular disks of charged surfactants in the monolayer on an aqueous solvent was calculated. Because charged surfactants of a two-dimensional monolayer in a homogeneous configuration would exhibit high electrical repulsion energy, they tend to segregate into heterogeneous clusters if it is possible. On the other hand, the line tension energy of the peripheries of the small clusters antagonizes the cluster formation. The screening of the electric repulsion by added ions in the aqueous solvent bed also affects the size of the cluster. The sum of the screened electric repulsive energy and the line tension energy was minimized to predict a circular disk. The theory is applied to-explain the observed monodispersed, nanometer-sized clusters of partially fluorinated long-chain acids on Langmuir monolayers as shown in the atomic force microscopy images by Kato and co-workers (Kata, T.; Kameyama, M.; Ehara, M.; Iimura, K. Langmuir 1998, 14, 1786).