Nanofilaments made of repeated units of multiwall carbon nanotubes (MWCNT) with a metal particle encapsulated at one end were synthesized by thermal chemical vapor deposition (CVD) of methane on Ni-Fe catalysts. The nanofilaments were characterized using scanning electron microscopy (SEM), low- and high-resolution transmission electron microscopy (TEM). A mechanism based on a sequential growth is here proposed to explain the structure. Using the structural information from the TEM micrographs, we demonstrate the existence of two different growth regimes depending on the size of the catalyst particle and estimate the critical carbon concentration in the catalyst particle to initiate the growth. (C) 2002 Elsevier Science B.V. All rights reserved.