Abstract Cobalt ferrite (CoFe2O4) nanoparticles have been prepared by chemical coprecipitation, mechanical milling and subsequent thermal treatment. Sodium chloride was added as a diluent during mechanical milling to avoid agglomeration of the particles. Thermal treatment of the as-milled powder at 600 °C produces well-crystallized CoFe2O4 nanoparticles with an average crystal size of 22.5 nm. The mechanism of nanoparticle growth was discussed.