Topographies of ultrathin organic films from octadecyltrichlorosilane (OTS) on mica surfaces pretreated with sodium ethoxide and an anchoring mechanism of the OTS molecules to the mica surface were investigated in ethanol by using an atomic force microscope (AFM). The film on the mica surface could be imaged repeatedly without being damaged when an applied force on the AFM tip was set at about 1 nN. The AFM image showed that the OTS molecules were reacted and arranged perpendicular to the surface to form a monolayer containing some pinholes, whose diameters range from several nanometers to about 100 nm. The monolayer on the mica surface was peeled off from the surface by the AFM tip at an applied force of 20 nN. The driving force to remove one molecule from the mica surface was calculated from the repulsive force exerted on the monolayer and was much smaller than that for breaking a typical covalent bond. These results indicate that almost all of the molecules in the monolayer are anchored to the mica surface not by covalent bonds but by physical adsorption.