With the aim of studying the structural and magnetic properties of cobalt doped magnetite nanoparticles         (Fe_3-xCo_xO4), several samples were synthetized by thermal decomposition method using different cobalt concentrations (0 – 5%) at different condition reflux time (30 – 120 min). In this work, we demonstrate that the synthetic parameters highly influence. Both, the morphology and the cobalt concentration, obtaining higher saturation magnetization values for 2 hours reflux. An exhaustive magnetic characterization by means of magnetization and electronic magnetic resonance have established conditions to improve the magnetic response of doped nanoparticles.