This paper outlines the fabrication and characterization of an immunosensor based on electrodchemical, biological, and nano-material techniques to achieve high sensitive detection of Escherichia coli (E.coli). The working electrode consists of E.coli antibody/antigen/horseradish peroxidase(HRP) labeled antibody immobilized on plasma-functionalized multiwalled carbon nanotube (pf-MWCNT) film. Immunosensors were characterized by cyclic voltammetry using 10 mM K 3Fe(CN)6/ 3 M KCl and 3,3′,5,5′- tetramethylbenzidine (TMB). This immunosensor shows very high sensitivity to E. coli O157:H7 and detection limit as low as 0.031 ng/ml. This is 100 times lower than that achievable with standard spectrophotometric enzyme-linked immunosorbent assay (ELISA) using the same immunochemicals.