Positive Cs ions are irradiated to a negatively biased substrate, which is covered with the dispersed single-walled carbon nanotubes (SWNTs) and immersed in the Cs plasma. Field emission type transmission electron microscopy (FE-TEM) and Z-contrast technique by scanning TEM (STEM) are used for the precise observation. FE-TEM gives high resolved images of structurally modified SWNTs such as irreversible bending of tube bundles, tube dislocation, and cutting due to the positive Cs ion irradiation. Moreover, this structural modification becomes severe with an increase in the acceleration energy of irradiating Cs ions. Especially, the Cs encapsulation inside SWNTs is directly observed, the configuration of which is confirmed to comprise three varieties by FEG-TEM and STEM. In addition to these experimental results, ab initio study is used to investigate the static stable configurations due to Cs ion irradiation. (C) 2003 Elsevier Science B.V. All rights reserved.