The; electron emission properties: of nanocrystalline diamond films were described. The nanometer structured diamond films were deposited by the microwave plasma assistance chemical vapor deposition method via continuous H+ ion bombardment. The grain size of nanocrystalline diamond films can be modified by means of changing the energy of bombarded ions. Scanning electron microscopy and Raman spectroscopy indicated the nanometer structure of the films. The results suggested that low-field electron emission and high emission current can be obtained from the films consisting of nanosized diamond grains.