Cold cathode electron field emission from nanocrystalline diamond thin films produced by microwave plasma-enhanced CVD with C-60 as the growth precursor has been observed. The lowest onset field obtained is about 2 V/mu m, and a 4 V/mu m field is required to emit a current density of 0.4 mA/cm(2). It has been found that hydrogen content of the plasma, which can be Varied over a wide range, strongly affects the microstructure and electron field emission properties of the nanocrystalline diamond thin films. This is an important new development which allows one to exercise powerful control over many properties of diamond films. Based on TEM characterization and field emission measurements, the effects of the structural defects and the orientations of the diamond grains on the field emission properties are discussed.