The preparation process, structure feature and field electron emission characteristic of diamond films on nanocyrstalline diamond coating by the CVD method were studied. The field electron emission measurements on the samples showed that the diamond films have lower turn-on voltage and higher field emission current density. A further detailed theory explanation to the results was given. The preparation process, structure feature and field electron emission characteristic of diamond films on nanocyrstalline diamond coating by the CVD method were studied. The field electron emission measurements on the samples showed the the diamond films have lower turn-on voltage and higher field emission current density A yet detailed theory explanation to the results was given.