A surface modified Fe3O4 nanoparticle with dextran, possessing a core-shell structure, was synthesized by vibration ball-milling of Fe3O4 with dextran in the solid state under vacuum. A combination of experimental results and density functional theory calculations demonstrate that the surface modified Fe3O4 nanoparticle with dextran was performed via a coupling reaction between a "naked" Fe+ atom of Fe3O4 mechano-cation and a "naked" O atom of dextran mechano-anion, viz., a "Fe:O" bond-formation. The naked Fe+ atom of Fe3O4 mechano-cation was produced by ionic scission of Fe-O bond of Fe3O4 and lost an "electron pair" under the ionic scission. The naked :O atom of dextran mechano-anion was produced by ionic scission of C-O bond comprising the alpha-1,6 glycosidic linkage of the dextran and gained the electron pair under the ionic scission. The Fe:O bond formation, viz., a novel type covalent bond formation, was achieved via an electron pair donation from the naked :O atom and its acceptance by the naked Fe+ atom. Consequently, a shared "electron pair" comprising the Fe:O bond was only donated from the naked :O atom. Our work demonstrates a novel method to form covalent bond formation between metal oxides and organic polymers, and provides a novel functionalized nanoparticle. (C) 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.