Polycrystalline diamond films consisting of submicron grains show efficient electron field emission properties, i.e. low emission threshold fields, high emission current and emission luminescent spot densities on indium tin oxide (ITO) coated glass anodes. After emission, we examined the emission sites on the surfaces of diamond crystals. Some micron-sized diamond crystals transformed into nanoparticle diamond, amorphous carbon, and graphite phases, as detected by micro-Raman spectroscopy in the local emission areas of the diamond films. The experiments indicate that nanoparticle diamond films with high sp(2) carbon phase content may be an efficient and stable cathode material for field emission display application.