The formation of carbon encapsulated Fe nanoparticles (Fe@C), in the process of growing single-/double-walled carbon nanotubes (S/DWCNTs) from methane was investigated by quantitatively analyzing the samples at different reaction time. We found that the ratio of Fe@C to the total Fe sustainably increased after the terminated growth of S/DWCNTs. We proposed that thin graphite layers produced in the middle stage of the reaction, as a mechanical barrier, covered the surface of MgO support and prevented Fe nanoparticles from contacting carbon source for the growth of S/DWCNTs via the root growth mode. These results suggested that the synthesis of desirable S/DWCNTs and undesirable Fe@C impurities was a consecutive process, not a paralleled one as proposed in many previous works. It provided useful information for the controlled synthesis of S/DWCNTs with high purity and high selectivity. (C) 2013 Elsevier B.V. All rights reserved.