Magnetic nanoparticles have been largely proposed as means of technological tools due to its value in different fields, especially in biomedicine. Herein, we present a robust, highly reproducible and low-cost method to obtain superparamagnetic magnetite nanoparticles (MNP-II) of about 15±5 nm diameter by thermal decomposition of [Fe(acac)3] in a one pot, two steps method. In the first step, magnetite nanoparticles (MNP-I) of lower size, 9±4 nm, with a saturation magnetization (Ms) of 65 emu/g and a coercive field (Hc) of 1 Oe are obtained. In the second step those particles MNP-I act as seeds for the final MNP-II which present a saturation magnetization of 70 emu/g and a coercive field of 12 Oe.