In this study a model based on thermodynamics was derived to calculate the critical and stable sizes of various elemental catalyst nanoparticles (Mo, Fe, Co, Ni and Au) formed from their nanofilms for the growth of carbon nanotubes. In the first step, the modeling of heterogeneous transformation of films into nanoparticleswas considered based on the size dependence of Gibbs free energy changes. At the second step, the reshaping of nanoparticles to nanorods was obtained based on the Laplace-Young equation. The proposed model was compared with the existing experimental data. The results of this study were in good agreement with the available experimental data. [ABSTRACT FROM AUTHOR]