Carcinoembryonic antigen (CEA), which is typically associated with certain tumors and developing fetus, is widely used as a clinical tumor marker for some familiar cancers. In this work, a simple and sensitive electrochemical CEA sensor was developed by employing immunoreaction. Gold nanoparticle-decorated graphene composites (Au-GN) were successfully synthesized based on the reduction of HAuCl4 in the presence of graphene. Horseradish peroxidase-labeled anti-CEA antibody (HRP-anti-CEA) and HRP were successively adsorbed on the Au-GN modified glassy carbon electrode. The stepwise assembly process of the immunosensor was characterized by cyclic voltammetry and electrochemical impedance spectroscopy. The introduction of CEA antigens on the electrode surface reduced the electrochemical response of the electron transfer mediator due to the strong steric effect. Under the optimized conditions, the peak current change derived from the differential pulse voltammetry (DPV) measurements (ΔIDPV) was proportional to the CEA concentration from 0.10 to 80 ng mL(-1) with a detection limit of 0.04 ng mL(-1) (S/N=3). In addition, this new protocol shows good selectivity, stability and reproducibility. The determination of CEA in human serum samples was performed and received in excellent accordance with the results determined by the enzyme-linked immunosorbent assay (ELISA).