By curving a rectangular diamondene, an sp 2/sp 3 composite carbon film, a diamondene nanotube (DNT) can be formed when the two straight edges are sewn together. In this study, thermal stabilities of DNTs are investigated using molecular dynamics simulation approaches. An interesting thermal shrinkage of damaged DNTs is discovered. Results indicate that DNTs have critical temperatures between 320 K and 350 K. At temperatures higher than the critical value, the interlayer bonds, i.e., the sp 3-sp 3 bonds, may break. The broken ratio of the interlayer bonds mainly depends on the temperature. For the DNT with a high broken ratio of interlayer bonds, it has thermal shrinkage in both the cross section and tube axis. The sp 2-sp 3 bonds in either the inner or the outer surface are much more stable. Even at 900 K, only a few sp 2-sp 3 bonds break. These properties can be used in the design of metamaterials.