In this Letter, the uncatalyzed edge growth of carbon nanotubes (CNTs) is investigated by first principles calculations and using tightbinding molecular dynamics simulations. Both heptagon and pentagon have been found to be the stable defective configurations at the growing edge. However, the activation energy of pentagon formation is rather high in comparison with that of heptagon. The heptagons can be annealed out by reaction with adatoms transported to the tube edge. The edge of the open-ended NT may remain stable, if it is fed by a sufficiently large surface diffusion flux. (c) 2006 Elsevier B.V. All rights reserved.