Nanodiamonds, doped with nitrogen, were successfully synthesized by using urea/methanol saturated solution as nitrogen source and in situ application of negative bias voltage during the growth period. Scanning electron microscope and Raman spectroscopic examinations reveal that increasing the urea/methanol ratio in the gas mixture markedly influences the field emission properties of diamond films, but insignificantly alters their structure and morphology. The diamond films thus obtained possess good electron field emission properties (E-0 = 2.35 V/mum, J(e) = 30.2 muA/cm(-2) and effective work function phi (e) = 0.028 eV). Converting the grains of diamond films from submicrons into nanosize (similar to 50 nm) via the application of bias voltage in situ further improves these field emission properties. The turn-on field decreases to E-0 = 1.88 V/mum, the field emission capacity increases to J(e) = 118.2 muA/cm(2) (at 6.5 V/mum applied field), and the effective work function decreases to phi (e) = 0.017 eV.