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Magnetic nanoparticles of cobalt ferrites CoxFe3-xO4 (x=1 or 2) have been obtained either by mechanical milling or thermal treatment of pre-prepared layered double hydroxide carbonate x-LDH-CO3. Mechanical milling of the 1-LDH-CO3 leads to the large-scale preparation of nearly spherical nanoparticles of CoFe2O4, the size of which (5 to 20 nm) is controlled by the treatment time. Core-shell structure with surface spin-canting has been considered for the nanoparticles formed to explain the observed hysteresis loop shift (from ZFC-FC) in the magnetic properties. Annealing treatment of the 2-LDH-CO3 below 673 K results in the formation of nearly spherical pure Co2FeO4 nanoparticles. At 673 K and above, the LDH decomposition leads to the formation of a mixture of both spinels phases Co2FeO4 and CoFe2O4, the amount of the latter increases with annealing temperature. Unusually high magnetic hardness characterized by a 22 kOe coercive field at 1.8 K has been observed, which reflects the high intrinsic anisotropy for Co2FeO4. Key words CoxFe3-xO4 (x =1, 2) nanoparticles . ferrite spinel . high energy ball-milling . high coercivity . Mössbauer Nano-sized materials are currently of great technological importance and consequently, one of the research subjects under intense investigation for their mechanical, electrical or chemical properties due to the fact that the properties of the nanophases being very different from those of the bulk material. Nanoparticles of materials are likely to improve dramatically the properties of the resulting material. Consequently, the synthesis of spinel ferrite nanoparticles has been intensively studied in recent years due to their potential application in high-density magnetic recording, microwave devices and magnetic fluids.