This study is concerning hybrid materials composed of the magnetite and the organic polymer such as dextran. They are useful for hyperthermia of cancer. In the preparation of this material, chemical structure or molecular weight of the added polymer is expected to affect ionic interaction between polymer and iron salts, and consequently the grain size and morphology of the prepared magnetite core. Therefore, we have synthesized magnetite-polymer hybrids using various polymers. Various polymers were dissolved in iron (II) chloride aqueous solution, and then NaOH aqueous solution was added to this mixed solution. As a result, in the case of neutral and cationic polymer crystalline magnetite was precipitated in the hybrid. On the other hand, in the case of anionic polyacrylic acid, lepidocrocite was precipitated rather than magnetite. It is known that the magnetite formation progresses through intermediate Fe(OH) 2 formation and oxidation of the Fe(OH)2 by dissolved O2. Therefore it is considered that tight ionic interaction is constructed between the iron ions and the carboxyl group in the polyacrylic acid to form a complex, and the Fe(OH)2 formation is inhibited. When the hybrid was prepared by addition of NaOH aqueous solution to iron (II) chloride solution, and subsequent addition of the different polymers, magnetite formation was not inhibited irrespective of kind of polymer. The present results indicate that crystalline structure of the magnetite phase in magnetite-polymer hybrid is strongly affected by the chemical structure of polymer additives or the order of addition.