Highly pure double-walled carbon nanotubes (DWCNTs) synthesized by a catalytic chemical vapour deposition method have a well-ordered bundle structure giving explicit diffraction peaks by synchrotron X-ray diffraction measurement. The changes of nanopore structural properties and water adsorptivity of DWCNTs with high-temperature heat treatment were investigated using molecular probe adsorption methods. It was founded that their nanoporosities and apparent hydrophilicities decreased with thermal annealing. However, a specific surface area of 275 m² g⁻¹ and the residual microporosity of more than 60% even after heat treatment at 2673 K suggest their unique applications.