Abstract: Objective To determine the physical and magnetic properties of superparamagnetic iron oxide(SPIO) nanoparitcle prepared in our laboratory and evaluate its possibility for use as contrast agents in magnetic resonance imaging(MRI).Methods The SPIO nanoparticle was obtained by means of classical coprecipitation in dextran solution and its size determined by electron microscopy and photon-correlation spectroscopy.The iron content was determined by phenanthroline photometry,and T2 values as well as relaxivity evaluated with a clinical MR system at 1.5T.Results Dextran-coated magnetite particles with a hydrodynamic diameter of 85.9 nm were prepared.The iron core size was 15 nm and the formation of Fe3O4 crystal in SPIO nanoparticles was confirmed by X-ray diffraction(XRD) analysis.These particles possessed some characteristics of superparamagnetic and show a smaller spin-spin relaxation,with relaxivity and saturation magnetization of 0.1567 mmol-1·ms-1 and 80 emu/g Fe,respectively.Conclusions A stable SPIO nanoparticle with a dextran coating have been developed,and in vitro evaluation of its physical and magnetic properities suggests its potential for use as the contrast agent in MRI.