An ultra-sensitive electrochemical immunosensor was successfully developed to accurately detect prostate specific antigen. To improve the performance of immunosensor, it was fabricated by using hybrid nano-structured electrodes of high performance chemically functionalized reduced graphene oxide (CRGO) and gold nanoparticles (AuNPs) followed by covalently immobilized monoclonal anti-PSA antibody. We optimized the immunosensor, under different conditions using cyclic voltammetry and differential pulse voltammetry. We obtained a low detection limit of 3 pg mL(-1) and wide linear range from 6 pg mL(-1) to 30 ng mL(-1), good reproducibility, selectivity, and stability. Finally, human serum samples were analyzed. The obtained recovery range was from 97% to 110% with relative standard deviation (RSD) of 3.88%. Thus, the developed immunosensor can provide a great potential in clinical and diagnostic applications. (C) 2017 The Electrochemical Society. All rights reserved.