By employing atomistic simulations based on empirical potential and density-functional theory, we study the irradiation of single-walled carbon nanotubes with a carbon ion. According to different impact locations on the surface of the nanotube, the incident threshold energies of defects formation are predicted to be 19, 35 and 45 eV, respectively. Moreover, the displacement threshold energy is investigated by using the collision dynamical method, and a reasonable value 17.59 eV is clarified by eliminating the thermal effect induced by the collision. Finally, the formation energy of a single vacancy is calculated by the ab initio method. (C) 2012 Elsevier B. V. All rights reserved.