Self-assembly enables exquisite control over the structures of various materials such as ultrathin organic films, [1][2][3][4] polymer films, [5][6][7] and inorganic nanoparticle assemblies. [8] Organic molecules are particularly suited for exploiting self-assembly because of the controllability of their molecular properties and intermolecular interactions using molecular design. Lateral patterning of organic materials has been performed using both top-down [6,9] and bottom-up [1][2][3][4][5]7] methods. However, construction of controllable nanostructures consisting of small organic molecules is difficult using bottom-up methods. In this Communication, we show that by using self-assembly, ultrathin films of small organic molecules are patterned with nanostructures, the sizes of which are controlled by adjusting intermolecular interactions. We demonstrate the formation of nanowires of controllable widths in phase-separated mixed monolayers. Organic and inorganic materials can be confined on the templates fabricated from phase-separated monolayers. Further, we report on the formation of spirals in threecomponent monolayers. We used the Langmuir-Blodgett (LB) technique for the fabrication of controlled nanostructures. Phase separation often occurs in mixed-Langmuir and LB films, the structures of which are governed by two competing interactions of line tension and dipole-dipole interactions. [1,3,10] Line tension favors the formation of large circular domains. Dipole-dipole interactions in monolayers is repulsive because each molecule is aligned in a parallel manner. Systematic variations in one of the interactions without changing the other should provide us with a means to control the phase-separated structures in monolayers. Along this line, we examined the phase-separated structures of two-component mixed LB films of fatty acid C k H 2k+1 COOH (HkA) and hybrid carboxylic acid C m F 2m+1 C n H 2n COOH (FmHnA), with systematic variations in k, m, and n. Dipoles that contribute dominantly to the phase-separated structures are those located at both ends of the molecules: [11] those due to methyl and carboxylic groups of HkA and those due to perfluoromethyl and carboxylic groups of FmHnA. Variations in alkyl and perfluoroalkyl chain lengths should not significantly affect the vertical component of the above dipoles as long as the molecular arrangement remains unchanged. Under these conditions, variations in the structures of the hydrophobic moieties of the molecules significantly affect only the line tension and not the dipole-dipole interactions. Figure 1 shows the change in the phase-separated structures of two-component LB films of H17A and FmH8A. Friction force microscopic observations show that the high and low parts consist of H17A and FmH8A, respectively. Circular domains of micrometer length scale form when m is small, showing the dominant role of line tension. In contrast, nanowires form when m is large, indicating the effect of dipole-dipole interactions.