Mo-coating on Si-substrate is observed to significantly improve the formation kinetics of diamond nuclei and the growth behavior of ultra-nanocrystalline diamond (UNCD) films. Contrary to the phenomenon that diamond nuclei are only scarcely formed on bare Si-substrates leading to incomplete coverage of UNCD grains, the diamond nuclei (and the UNCD grains) are found to cover the whole surface of Mo-coated Si-substrates. While the Mo-coating markedly enhances the nucleation of diamonds, it degrades the electron field emission (EFE) properties of UNCD films. It is attributed to the conversion of the conducting Mo-metallic film into a resistive Mo2C layer during the microwave plasma CVD process. A sufficiently thick Mo-layer is found to enhance the nucleation of diamonds, while minimizing deleterious effect on their EFE properties with an improved turn-on-field of 13 V/μm and a current density of 55 μA/cm2 at the applied field of 30 V/μm.