We directly imaged a monolayer of fatty acids adsorbed onto a graphite substrate from binary mixtures of myristic acid and behenic acid using a scanning tunnelling microscope (STM). The overall STM images of a monolayer of pure fatty acids exhibit an alternate structure composed of bright bands corresponding to the alkyl chains and chained dark regions at the carboxyl groups. In the STM images of a monolayer of binary mixtures, the entire area is filled with 2 kinds of the bright bands, i.e., there are the short and long bands corresponding to the alkyl chains of 2 kinds of fatty acids. The ratio of the short and long bands in the images suggests that the adsorption of fatty acid with a longer alkyl chain on the graphite substrate is preferential. Moreover, a dynamic exchange of 2 kinds of the bands was often imaged between 2 successive STM images over several tens of seconds. This fact indicates the existence of molecular motion at the interface between a solution and the graphite substrate.