A surfactant, cetyltrimethylammonium bromide (CTAB), is found to assist the self-assembly of fatty acids on flat solid surfaces in aqueous solutions. Structural studies using an atomic force microscope show that self-assembled structures are planar on a mica surface. Quantifying the self-assembled materials shows that the amount is enough to account for more than 10 molecular layers. Comparative studies of several area-pressure isotherms using a Langmuir trough under different conditions indicate that the self-assembly causes the formation of stable surfactant-fatty acid molecular complexes.