The formation energetics and stability of n-member rings at the end of small zigzag carbon nanotubes are investigated using k-space tight-binding molecular dynamics simulations (TBMDs). It is found that at 1100 degreesC the initial dangling atoms at the end of a small (n, 0) zigzag carbon nanotube can transform into an n-member ring if the tube is larger than a (3,0) tube but smaller than a (11,0) tube. For zigzag tubes larger than (10,0) the large strain associated with the n-member ring prevents the formation of the ring at the end of the tube. It is shown that the 6-member ring formed at the top of a (6,0) tube is the most stable one, and that the formation of the ring at the top of small zigzag tubes may introduce new ring states around the Fermi level. (C) 2002 Published by Elsevier Science B.V.