[en] Cobalt ferrite nanoparticles were obtained by the sol-gel method at several annealing temperatures: 400, 450, 500, 550 and 600 deg. C. X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed the formation of the spinel phase with a nanoparticle size in the 17-26 nm range as function of the annealing temperature. The Moessbauer spectra at room temperature showed the presence of a partial inverse spinel structure. Saturation magnetization and the coercive field are strongly dependent on the annealing temperature and they can be associated with variations of the nanoparticles size. Microwave power absorption (MPA) (dP/dH) measurements were carried out as a function of DC field (H_DC) in asymmetric sweeps in the 0 kOe≤H_DC≤9 kOe range, at X-band (9.4 GHz), for all annealing temperatures. The large hysteresis in the MPA is due to interparticle interaction associated with its demagnetizing-like nature