Transmission electron microscopy (TEM) showed that Te in the inner space of carbon nanotubes (CNTs) with inner diameters of less than 1.7-2.0 nm adopted an atomic chain structure that formed double helices near the CNT wall. In contrast, crystalline Te consisting of spiral chain molecules with a ternary screw axis was observed in CNTs with inner diameters greater than 1.7-2.0 nm. The structural transformation related to the inner diameter can be attributed to different interactions between Te and the CNT wall depending on the inner diameter. (C) 2015 Elsevier B.V. All rights reserved.