Cobalt ferrite (CoFe2O4) possesses excellent chemical stability, good mechanical hardness and a large positive first-order crystalline anisotropy constant, which made this ferrite a promising candidate for magneto-optical recording media. In addition to precise control on the composition and structure of CoFe2O4, the success of its practical application will depend on the capability of controlling particle size at the nanoscale. This size-controlled synthesis approach became possible by modifying the oversaturation conditions during ferrite formation in water. Optimum oversaturation was achieved by monitoring of the feeding flow-rate of reactant solutions. X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) analyses confirmed the formation of the ferrite structure after a reaction time as short as 5 min. M-H measurements verified the strong influence of synthesis conditions and crystal size on the magnetic properties of ferrite nanocrystals. The coercivity values increased from 210 up to 5840 Oe under optimum synthesis conditions. © 2008.