Iron nanoparticles were synthesized in a microwave-plasma reactor by thermal decomposition of iron pentacarbonyl Fe(CO)5 (IPC) diluted in Ar. The pressure in the plasma discharge was kept at 30 mbar. The particle size was measured in-situ by a particle mass spectrometer (PMS). Depending on the reaction conditions, mean particle diameters between 4 and 10 nm were observed. The influence of precursor concentration, flow rate and microwave power on the particle size were investigated. In the modeling the microwave energy was treated as a spatially distributed energy source term that results in an increase in the gas temperature. The particle formation and evolution were simulated using the sectional aerosol model introduced by Hounslow et al. In addition to the particle volume, the sectional model was also extended to the particle surface area. The model takes into account the thermal decomposition of the precursor, nucleation, coagulation, coalescence, convection, and diffusion effects. The simulations show good agreement with the experiments, although the details of the plasma process were not considered. CFD calculations were carried out with Fluent 6.2.