Anisotropic Janus particles with strong magnetic iron oxide domain and polymer microsphere domain were prepared with large scale in one batch through a facile method via the solvothermal method in the presence of poly(methylenebisacrylmide-co-methacrylic acid) (P(MBA-co-MAA)) microspheres. The coordination interaction between the carboxyl acid groups and inorganic species played an essential role during the formation of uniform P(MBA-co-MAA)/iron oxide core-shell microspheres via the controlled hydrolysis of ferric chloride in the initial stage. The formation of P(MBA-co-MAA)/Fe3O4 Janus particles may be originated from a thermodynamic incompatibility between the P(MBA-co-MAA) domain and the Fe3O4 domain during the partial reduction and crystallization of inorganic component to lower the interfacial energy change via reducing the interfacial area between the polymeric and inorganic magnetic domain. Further, the sizes of the inorganic magnetite domains were well controlled via altering the ratio of P(MBA-co-MAA) microspheres to ferric chloride inorganic precursors. (c) 2012 Elsevier Inc. All rights reserved.