A novel multifunctional reduced graphene oxide (RGO)/Fe3O4/PDDA/CdSe nanocomposite has been successfully prepared and used as probe for an electrochemiluminescence (ECL) immunosensor of human interleukin-6 (IL-6). The prepared RGO/Fe3O4/PDDA/CdSe nanocomposite was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible (UV-vis) absorption spectroscopy, and a vibrating sample magnetometer (VSM). This nanocomposite shows magnetism, ECL properties, and good biocompatibility. Furthermore, the RGO/Fe3O4/PDDA/CdSe nanomaterial was applied to develop an ultrasensitive ECL immunosensor for detection of IL-6. The ECL intensity decreased linearly with the IL-6 concentrations in the range 0.002 to 20 ng/mL. Furthermore, the developed ECL immunosensor with high sensitivity, stability and excellent reproducibility can be used for detection of real samples, which make it possible to be used in clinical detection. (C) 2016 Published by Elsevier Ltd.