Cathodic　electrochemiluminescence　(ECL)　is　firstly　observed　at　a　carbon　oxide　covered　glassy　carbon　(C/CxO1-x)　electrode　as　a　large　cathodic　pulse　polarization　is　applied.　This　insulating　carbon　oxide　(CxO1-x)　film　is　constructed　on　a　glassy　carbon　(GC)　substrate　by　electrochemical　oxidization　in　basic　media.　The　film　properties,　such　as　the　composition　of　carbon　and　oxygen,　and　the　thickness　as　well,　can　be　controlled　by　the　potential　and　the　duration　in　the　oxidizing　process.　X-Ray　photoelectron　spectroscopy　(XPS)　studies　show　that　carbonyl　and　carboxyl　dominate　at　the　oxidized　surface,　to　which　antibodies　can　be　covalently　bound.　The　specific　immunoreaction　between　antigen　(Ag)　and　antibody　(Ab)　resulted　in　a　decrease　in　the　ECL　intensity,　thus　creating　an　interesting　basis　for　the　development　of　a　label-free　cathodic　ECL　immunosensor.　As　an　example,　human　IgG　(hIgG)　was　sensitively　determined　in　the　concentration　range　of　0.01-100　ng　mL-1,　and　the　detection　limit　was　ca.　1.0　pg　mL　-1　(S/N　=　3).　In　addition,　the　content　of　hIgG　in　human　serum　has　been　assayed　by　the　developed　immunosensor　and　a　commercially　available　immune　turbidimetry　method,　respectively,　and　consistent　results　were　obtained.　The　prepared　immunosensor　provides　a　promising　approach　for　the　clinical　determination　of　IgG　levels　in　human　serum,　because　it　is　simple,　rapid,　highly　sensitive,　specific,　and　without　the　need　of　tedious　labeling　operations.　©　2010　The　Royal　Society　of　Chemistry.