The single or multienergy nitrogen (N) ion implantation (MENII) processes with a dose (4×1014 ions/cm2) just below the critical dose (1×1015 ions/cm2) for the structural transformation of ultrananocrystalline diamond (UNCD) films were observed to significantly improve the electron field emission (EFE) properties. The single energy N ion implantation at 300 °C has shown better field emission properties with turn-on field (E0) of 7.1 V/μm, as compared to room temperature implanted sample at similar conditions (E0=8.0 V/μm) or the pristine UNCD film (E0=13.9 V/μm). On the other hand, the MENII with a specific sequence of implantation pronouncedly showed different effect on altering the EFE properties for UNCD films, and the implantation at 300 °C further enhanced the EFE behavior. The best EFE characteristics achieved for the UNCD film treated with the implantation process are E0=4.5 V/μm and current density of (Je)=2.0 mA/cm2 (at 24.5 V/μm). The prime factors for improving the EFE properties are presumed to be the grain boundary incorporation and activation of the implanted N and the healing of induced defects, which are explained based on surface charge transfer doping mechanism.