The improvement on the electron field emission (EFE) properties of duplex-structured diamond films by N-ion implantation/post-annealing processes was investigated. The duplex-structured diamond films were synthesized by a two-step microwave plasma enhanced CVD process. Transmission electron microscopy (TEM) examinations reveal that all the as-prepared, N-ion implanted and post-annealed diamond films contained large microcrystalline-diamond (MCD) aggregates sparsely distributed among the matrix of ultra-small diamond grains. While the granular structure of the MCD aggregates was insignificantly modified due to the N-ion implantation/post-annealing processes, that of the UNCD regions was markedly altered. The EFE process for the MCD/UNCD films can be turned on at (E0)MCD/UNCD = 8.21 V/μm, which is even smaller than the E0-field for the UNCD films ((E0)UNCD = 13.34 V/μm). These N-ion implanted/post-annealed diamond films attained an EFE current density of (Je)MCD/UNCD = 0.4 mA/cm2 at an applied field of 20.0 V/μm that is even larger than the Je-value for the UNCD films ((Je)UNCD < 0.05 mA/cm2 at the same applied field). Presumably, the enhanced EFE properties are resulted from the presence of nano-graphites in the small-grain region of MCD/UNCD films that form an interconnected path, facilitating the transport of electrons.