Bio-compatible and superparamagnetic Fe3O4@Polydopamine nanocomposites with well-defined core/shell nanostructures have been successfully synthesized by using an in situ self-polymerization method. The size of the core/shell product can be controlled by varying the size of the central Fe3O4 core, and different thicknesses of the PDA shells are obtained by tuning the dopamine monomer concentration. The morphology, phase composition and crystallinity of the as-prepared nanocomposites have been characterized by transmission electron microscopy (TEM) and powder X-ray diffraction (XRD). The 5-dimethylthiazol-2-yl-2,5-diphenyl-tetrazolium bromide (MTT) cytotoxic analysis shows that the product is biocompatible, with a low toxicity. These unique core/shell materials would be applied in catalyst supports or drug delivery. (c) 2011 Elsevier B.V. All rights reserved.