A new process was developed for synthesis of magnetic polymer microspheres which can be used as hyperthermia carrier for cancer therapy. Polymer microspheres containing magnetic nanoparticles were successfully prepared by in situ formation technique of magnetite combination with double emulsion method. Poly (styrene-co-2-hydroxyethyl methacrylate) (PS-HEMA) dissolved into ethyl acetate (EA) was employed as oil phase (O). Ferrous chloride (Fe2+) and ferric chloride (Fe3+) aqueous solution was used as an internal water phase (W-1), which was dispersed into polymer solution to form primary emulsion of W-1/O. Then, this emulsion was further dispersed into external water phase (W-2) to obtain double emulsion of W-1/O/W-2. Finally, the alkaline solution of ammonia was added into this system, and the ammonia continuously diffused into the internal water phase (W-1) to react with Fe2+ and Fe3+. As a result, Fe3O4 magnetic nanopartides were formed in situ in the internal water phase. Various process parameters including Fe2+/Fe3+ concentrations in internal water phase, PS-HEMA concentrations, Span85 amounts in oil phase and volume fractions of pre-solidification solution affected stability of primary emulsion, loading efficiency of magnetic nanoparticles, morphology and magnetization of microspheres, which were systematically investigated in this study. Compared with the traditional method, the magnetic polymer microspheres prepared by in situ magnetization technique demonstrated excellent encapsulation efficiency (26.1%) and high magnetization (12.2 emu/g). Therefore, the as-prepared magnetic polymer microspheres show good potential for cancer therapy. (C) 2012 Elsevier B.V. All rights reserved.