Cobalt ferrite particles with diameters ranging from a few micrometer to about 15 nm were synthesized using a modified oxidation process. The fine control of the particle size was achieved by introducing various concentrations of Fe3+ ions at the beginning of the reaction. Among the particle sizes obtained by using this method, particles with a grain size of about 36 nm showed a magnetization (Ms) of 64 emu/g and a maximum coercivity (Hc) of 2020 Oe at room temperature. The corresponding squareness ratio was found to be 0.53. © 2003 Elsevier Science (USA). All rights reserved.