Superparamagnetic iron oxide (SPIO) nanoparticles were used more and more universally in biomedical research recently. In order to develop a suitable immuno-magnetic probe which can be applied in antigen-antibody based immunoassay, such as tumor marker or cardiovascular marker detection, SPIO capped by dextran was synthesized with chemical coprecipitation method. With sodium periodate method, hydroxyl groups of dextran were oxidated to aldehyde groups, thus conjugated with antibodies by covalent bonds. Western blot pattern illustrated that structure integrity of the most antibodies was kept well, harmful influence given by oxidation of conjugation was very finite. Both immune reactivity and antigenicity of antibodies in the SPIO based immunoprobes were retained effectively. This probe has a potentially application in SPIO based novel suspension array for its superparamagnetic property that can be separated under gradient magnetic field.