Immunoassays, with high selectivity and affinity of antibody molecules to their corresponding antigens, have been widely used in clinical diagnosis. Electrochemical immunoassays, with simple instrumentation, high sensitivity, fast response, low detection limit, wide dynamic range have widely been used as predominant analytical techniques for biomolecules detection. In this work a novel and highly sensitive electrochemical immunosensor was developed for detection of cancer biomarker prostate specific antigen (PSA) based on immobilization of PSAantibody (anti-PSA) onto effective and robust nanocomposite containing multiwalled carbon nanotube (MWCNTs)/ionic liquid(IL) 1-buthyl-methylpyrolydinium bis (trifluromethyl sulfonyl) imide [C4mpyr][NTf2] and thionine.