Enhanced electron field emission (EFE) properties for ultrananocrystalline diamond (UNCD) films grown on silicon substrate were achieved, especially due to the high dose N ion implantation. Secondary ion mass spectroscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy measurements indicated that the N ion implantation first expelled H-, induced the formation of disordered carbon (or defect complex), and then induced the amorphous phase, as the ion implantation dose increased. The postimplantation annealing process healed the atomic defects, but converted the disordered carbon to a stable defect complex, and amorphous carbon into a more stable graphitic phase. The EFE characteristics of the high dose (≫1015ions/cm2) ion-implanted UNCD were maintained at an enhanced level, whereas those of the low dose (≪1014ions/cm2) ion-implanted ones were reverted to the original values after the annealing process. Ion implantation over a critical dose (1×1015ions/cm2) was required to improve the EFE properties of UNCD films.