Carbon was coated on the surface of molybdenum-grids by ion beam assisted deposition(IBAD).Electron emission characteristics from the grids with and without carbon contaminated by active electron-emission substance of the cathode were measured using analogous diode method.The results show that electron-emission from the grid with carbon was much less than that from the grid without carbon.The cathode-emission substance deposited on the carbon surface of the grid was analyzed by XPS and electron probe (EPMA) and the mechanism for suppression of electron emission of the grid with carbon is discussed.