The formation energy of single Stone-Wales (SW) defect with possible two different orientations in (5,5) single-walled carbon nanotubes (SWNTs) has been investigated using Hartree-Fock and MP2 methods and B3LYP functional. The formation energies computed at the B3LYP/6-31G(d) level are in good agreement with those obtained at the MP2/6-31G(d) level. A 90 degrees rotation of an axial C-C bond is slightly more preferred than the circumferential C-C bond in forming the Stone-Wales defect in (5,5) SWNT. The reactivity of the C-C bond shared by two heptagons in the SW defect of SWNTs depends on the orientation of the SW defect. (c) 2006 Elsevier B.V. All rights reserved.