The electron field emission (EFE) properties of Si-nanowires (SiNW) were improved by coating a UNCD films on the SiNWs. The SiNWs were synthesized by an electroless metal deposition (EMD) process, whereas the UNCD films were deposited directly on bare SiNW templates using Ar-plasma based microwave plasma enhanced chemical vapor deposition (MPE–CVD) process. The electron field emission properties of thus made nano-emitters increase with MPE–CVD time interval for coating the UNCD films, attaining small turn-on field (E0 = 6.4 V/μm) and large emission current density (Je = 6.0 mA/cm2 at 12.6 V/μm). This is presumably owing to the higher UNCD granulation density and better UNCD-to-Si electrical contact on SiNWs. The electron field emission behavior of these UNCD nanowires emitters is significantly better than the bare SiNW ((E0)SiNWs = 8.6 V/μm and (Je)SiNWs < 0.01 mA/cm2 at the same applied field) and is comparable to those for carbon nanotubes.