Nitrogen containing nanocrystalline diamond films have been synthesized by microwave plasma enhanced chemical vapour deposition using N₂/CH₄ as the reactant gas. The films prepared under different N₂/CH₄ flow ratios were studied by scanning electron microscopy, transmission electron microscopy, x-ray diffraction and secondary ion mass spectroscopy. Their field emission characteristics, i.e. both the distribution of the emission sites and current-voltage characteristics, were recorded using a transparent anode technique. It was found that the grain size of films prepared under different conditions varies and has a relationship with their turn-on fields. A possible explanation is given, which proposes that nanocrystalline boundary induced defect states might affect the field emission characteristics.