Ultrafine iron powder was synthesized in an atmospheric-pressure radio-frequency plasma reactor by injecting relatively course iron powder into the plasma, where it evaporated. The renucleated iron particles were characterized by means of a sampling capillary and dilution system interfaced to an electrical aerosol analyzer, a condensation nucleus counter and an electrostatic aerosol sampler. Volume-mean particle diameters for samples obtained near the downstream end of the reactor ranged from about 20 to 70 nm, with particle size increasing as the feed rate of injected iron powder was increased. A two-dimensional numerical model was developed, which solved the plasma conservation equations to predict temperature and velocity distributions, heating and evaporation of the injected iron powder, nucleation and growth of iron particles, and particle transport by convection, diffusion and thermophoresis. Mean particle diameters predicted by the model were in good agreement with the experimental data, although the data indicated broader size distributions and flatter radial profiles of particle concentration than predicted by the model.