A new method for the preparation of magnetic nanospheres is reported. It involved the dispersion of an aqueous phase containing Fe3+, Fe2+, or/and Ni2+ ions into droplets in an organic medium with an amphiphilic block copolymer, which was synthesized through atom transfer radical polymerization at a lower temperature (70 degrees C) and was then sulfonated by sodium hydrogen sulfite in tetrahydrofuran as a surfactant. A reduction was carried out in the water pool as a microreactor, which resulted in the yielding of polymer/magnetite particles. The size of the prepared spheres could be tuned up by variation of the preparation conditions, including the stirring speed, copolymer content, and so on. The average diameter was homogeneous and was about 10 nm. The magnetic nanospheres, whose saturation magnetization of magnetic nanoparticles of 3.8 X 10(-3) emu/g was less than their bulk counterparts, exhibited characteristics of superparamagnetism. (C) 2008 Wiley Periodicals, Inc.