In this study, laser-induced incandescence (LII) diagnostic technique was applied for iron-based nanoparticle (NP) sizing during the floating chemical vapor deposition (CVD) synthesis of carbon nanotubes (CNTs). Transmission electron microscopy (TEM) was used to characterize the nature and size of NPs. The LII signal was simulated by taking into account the carbon-encapsulated iron NP density, heat capacity, size distribution, etc. A detailed sensitivity and uncertainty of the key parameters on the evaluated particle size for this model has also been estimated. Using the developed approach, the evolution of NPs in the gas phase along the reactor axis was investigated at 650, 750 and 850 °C. It was found that the evaluated sizes from LII signals were in good agreement with the ones obtained by TEM measurements. The NP size is highly dependent on the temperature under the studied conditions but it does not show obvious difference along the reactor axis. This study reveals an important LII application prospect to understand the catalyst particle behaviors for better control over CNT growth during the floating CVD process.