The magnetic properties in nanoscale ferrite materials are strongly dependent on the crystal size, morphology, and cation distribution in the lattice. The present work addressed the synthesis of Co-substituted ferrite nanocrystals were attempted at various staring Fe:Co mole ratios (3:1, 2:1, 1.7:1, and 1.4:1) and the corresponding structural and magnetic properties determined. The synthesis of the ferrite powders was carried out by the conventional and modified coprecipitation method. The later consists of contacting the metal ions solution with hydroxide ions at controlled flow-rates to promote the heterogeneous nucleation, where earlier produced ferrite nuclei will act as seeds, and hence crystal growth. The actual Fe:Co mole ratios in the as-synthesized samples were determined by energy dispersive X-ray spectroscopy (EDS). Obtained nanocrystals were also characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM), and Mössbauer spectroscopy techniques. Cobalt ferrite nanocrystals ranging between 11 and 19 nm exhibited coercivity values between 114 and 4412 Oe. The variation in coercivity values of cobalt ferrite nanocrystals with different compositions was mainly attributed to the remarkably enlargement of crystal size under flow-rate controlled synthesis conditions, and the particular distribution of cations between A-and B-sites in addition to surface anisotropy contribution. © 2011 Elsevier B.V. All rights reserved.