Ultra-nano-crystalline diamond (UNCD) nano-emitters were prepared by coating UNCD films on the tip of silicon nanowire (SiNW) templates by microwave plasma-enhanced chemical vapor deposition process. The electron field emission properties of the UNCD/SiNW nano-emitters varied markedly with the pre-seeding process for the SiNW-templates. The direct ultrasonication process is more efficient in the formation of the diamond nuclei than the carburization/ultrasonication process, yielding UNCD/SiNWs nano-emitters with better electron field emission properties. The electron field emission can be turned on at (E0)UNCD/SiNW4 = 3.75 V/μm, yielding a large electron field emission current density of (Je)UNCD/SiNW4 = 11.22 mA/cm2 at an applied field of 9.75 V/μm. These characteristics are significantly better than those of bare SiNWs or planar UNCD films.