A method based on a microemulsion route is presented to obtain magnetic nanoparticles of different sizes; offering several advantages with respect to other similar methods. It consists basically of the development of a microemulsion route above room temperature. Magnetic iron oxide particles with sizes ranging from 2 to 8 nn are obtained. The as-obtained product presents defined X-ray diffraction patterns, without the complications of working in an inert atmosphere or with extra amounts of Fe2+, thereby eliminating the need for further steps in order to achieve good crystallization of the material.