Nucleation of carbon nanotubes is considered to start with the supersaturation of carbon atoms dissolved in a catalyst. In chemical vapor deposition synthesis, it is believed that condensation of the source gas terminates the growth. However, some experimental studies have suggested that termination is primarily affected by catalyst behavior. We modified the potential functions and observed the behavior of carbon-iron mixed nanoparticles by a molecular dynamics method and found that the iron fraction melted at the equivalent temperature of growth termination. Thus, we concluded that the catalyst fusion prevented nucleation that would normally occur as a consequence of solution supersaturation. (C) 2014 Elsevier B.V. All rights reserved.