The classical molecular dynamics (MD) simulation at about 3000 K shows that the formation of an armchair carbon nanotube (CNT) nucleus from graphite flakes occurs within 10 ps by the zipper-like mechanism and it consists of only hexagonal structures without defects, while the transformation into a zigzag CNT nucleus needs more time than 50 ps and it involves many defects such as pentagons, heptagons and octagons. The transformation energy from graphite into CNT nucleus is found to be related with both the surface plane energy and the energy involved in bending from graphite sheet to tube. (C) 2003 Elsevier B.V. All rights reserved.