Abstract: Carbon nanotubes (CNTs) are filled with Na125I after purification and opening. The behaviour of filling, washing and release of Na125I from CNTs are investigated. CNTs samples filled with NaI under optimum filling condition are characterized by HREM, SEM and EDS. The results indicate that the Na125I are filled into the CNTs and can release slowly. It shows that the radioactive tracer technique is effective for the study of filling and release behaviour of CNTs.