The wettability of individual carbon nanotubes (CNTs) with <20 nm diameter was experimentally evaluated on the basis of the Wilhelmy method via atomic force microscopy to visualize the effect of the nanometer-scale curvature of a solid surface on the solid-liquid and solid-vapor interface tensions. The experimental results showed the deviation of force owing to wetting for diameters <10 nm. In particular, for nanotubes with <4.5 nm diameter, the wettability tended to differ from its previously predicted behavior. The diameter-dependent wettability seen below 10 nm can be attributed to the interaction between the curved solid surface and the curved liquid adsorption layer formed on the CNT surface. In the scale where the radius of curvature is less than 5 nm, the thermodynamics perspective may not be valid, and thus, an atomistic perspective must be considered.