Large-area ultra-nanocrystalline diamond-coated silicon nanowire (UNCD/SiNW) field-emitter arrays were prepared by the deposition of ultra-nanocrystalline diamond (UNCD) on the tips of arrays of silicon nanowires (SiNWs) with uniform diameters. The electron field-emission (EFE) behavior of UNCD/SiNW arrays as well as that of the SiNW arrays has been observed. The SiNWs exhibit good electron field-emission properties with turn-on fields (E 0) of about 7.6 V μm-1, which is superior to the EFE properties of planar-silicon materials. The turn-on fields are related to the diameter of the SiNWs. Coating the SiNWs with a UNCD film further improves their EFE properties. The threshold field for attaining Je = 0.1 mA cm-2 EFE current density is 16.0 V μm-1 for bare SiNWs and 10.2 V μm-1 for UNCD/SiNWs. The improvement in EFE properties due to the UNCD coating is presumably due to the lower work function of field emission of the UNCD materials, compared to that of the silicon materials.