For the purpose of improving the electron field emission properties of ultra-nanocrystalline diamond (UNCD) films, nitrogen species were doped into UNCD films by microwave plasma chemical vapor deposition (MPCVD) process at high substrate temperature ranging from 600° to 830 °C, using 10% N2 in Ar/CH4 plasma. Secondary ion mass spectrometer (SIMS) analysis indicates that the specimens contain almost the same amount of nitrogen, regardless of the substrate temperature. But the electrical conductivity increased nearly 2 orders of magnitude, from 1 to 90 cm− 1 Ω− 1, when the substrate temperature increased from 600° to 830 °C. The electron field emission properties of the films were also pronouncedly improved, that is, the turn-on field decreased from 20 V/μm to 10 V/μm and the electron field emission current density increased from less than 0.05 mA/cm2 to 15 mA/cm2. The possible mechanism is presumed to be that the nitrogen incorporated in UNCD films are residing at grain boundary regions, converting sp3-bonded carbons into sp2-bonded ones. The nitrogen ions inject electrons into the grain boundary carbons, increasing the electrical conductivity of the grain boundary regions, which improves the efficiency for electron transport from the substrate to the emission sites, the diamond grains.