Boron-doped microcrystalline diamond (BMD) and nanocrystalline diamond (BND) thin films were grown on Si substrates by microwave-assisted chemical vapor deposition, and their field emission properties were evaluated. BND exhibited a lower turn-on field and higher field enhancement factor than BMD. Furthermore, in a long-term emission stability test, BND showed only a 4% increase in the current density after 12 h of emission, whereas the current density of BMD decreased by - 59%. These results indicate that BND is a more stable and viable current emitter than BMD.