Nano magnetic particles are extensively used in both clinical diagnostics and basic researches for its distinct features, such as cell separation, immune recognition, contrast reagents in Magnetic Resonance Imaging (MRI). This paper proposes a method of preparing magnetic nanoparticles in the aqueous phase, which can be heated in an alternating magnetic field. Through orthogonal experiments, the relationship between particle diameter and reaction condition (dose of ammonia, iron salt, dextran and temperature) is optimized. The size of magnetic nanoparticles and their distribution, the relationship between dextran shell and iron oxide core, the magnetic characteristics are measured. The dextran coated iron oxide nanoparticles prepared in this way can be stored in room temperature for several months without any change of properties.