Almost all carbon nanotube synthesis methods use seeded catalyst nanoparticles, either in the gas or liquid, as the initiator of carbon nanotube growth. However, the detailed mechanism of nucleation and surface growth of the catalysis nanopartilces is not well understood, although they influence diameter and possibly the length of final carbon nanotubes. Here as our first attempt, we created a simple two-equation (monodisperse) model that can simulate the formation process of nanoparticles, and simulated the formation process of iron nanoparticles from ferrocene used in a chemical vapor deposition (CVD) reactor. This model coupled with computational fluid dynamics (CFD) simulation was applied to study the effect of gas temperature, gas flow rate, feedstock concentration, and reactor length on nanoparticle formation.