AbstractWater‐dispersible carbon nano‐test tubes (CNTTs) with an inner and outer diameter of about 25 and 35 nm, respectively, were prepared by the template technique and then their inner carbon surface was selectively oxidized to introduce carboxy groups. The adsorption behavior of DNA molecules on the oxidized CNTTs (Ox‐CNTTs) was examined in the presence of Ca²⁺ cations. Many DNA molecules are attracted to the inner space of Ox‐CNTTs based on the Ca²⁺‐mediated electrostatic interaction between DNA phosphate groups and carboxylate anions on the inner carbon surface. Moreover, the total net charge of the DNA adsorbed was found to be equal to the total charge of the carboxylate anions. This selective adsorption into the interior of Ox‐CNTTs can be explained from the fact that the electrostatic interaction onto the inner concave surface is much stronger than that on the outer convex surface. On the other hand, the desorption of DNA easily occurs whenever Ca²⁺ cations are removed by washing with deionized water. Thus, each of Ox‐CNTTs works well as a nano‐container for a large amount of DNA molecules, thereby resulting in the occurrence of DNA enrichment in the nanospace.