We synthesized crystalline Fe-Ni nanoparticles with various particle sizes by reducing NiCl2, and FeCl2, vapors with hydrogen simultaneously. To control the primary particle size, processing variables of evaporator temperature, reaction zone temperature, and total gas flow rate were varied. The nanoparticles were nearly spherical and formed directional linkage between them due to magnetic interaction. The XRD patterns and elemental compositions measured by EDS showed that the Fe-Ni nanoparticles were mainly composed of cubic FeNi3. With various evaporation temperatures from 800 to 900 degrees C, the reactant concentrations were estimated to range from 7.94 x 10(-6) to 2.68 x 10(-5) mol/l, which resulted in the specific surface area and Sauter diameter of the particles from 11.1 to 8.8 m(2)/g and from 65 to 82 nm, respectively. The geometric standard deviations of the primary particle sizes obtained from TEM micrographs ranged from 1.24 to 1.27, indicating very narrow particle size distribution. The increase in the reaction temperature from 850 to 950 degrees C led to the reduction of the Sauter diameter from 69 to 63 nm. As the total gas flow rate decreased from 5 to 3 l/min, the Sauter diameter increased from 56 to 69 nm. (c) 2005 Elsevier B.V. All rights reserved.