A disposable, magnetism-controlled, and amperometric immunosensor was fabricated for the determination of C reactive protein (CRP). Firstly, Fe3O4 (core)/Au (shell) nanoparticle (GMP)–coated multiwalled carbon nanotubes (MWCNT–GMP) were prepared. Then, monoclonal antibody (anti-CRP) was immobilized on MWCNT–GMP to prepare MWCNT–GMP/anti-CRP composite magnetic probes. At last, the probes was adsorbed on the surface of N,N''-bis-(2-hydroxy-methylene)-o-phenylenediamine cobalt (CoRb) modified, screen-printed carbon electrodes through external magnetic field.The amount of CRP in the serum sample was determined by one-step immunoassay. When different concentrations of CRP were added into25μL of phosphate-buffered solution (pH7.0) containing10-4M H2O2, the percentage of DPV cathodic peak current decrease (CR%) exhibited a linear relationship with the concentration of CRP in the range of 0.3–100 μg L-1.The detection limit was 0.16 μg L-1(3б).The immunosensor was used to determine CRP in serum samples of patients with heart diseases, and the results were consistent with those of the traditional ELISA method. The proposed amperometric immunosensor was sensitive, rapid, magnetic field–controlled, and disposable; therefore, it could be used to determine even traces of CRP in the blood serums of patients with heart diseases.