Nano-diamond particles are co-deposited on Ti substrates with metal (Ti/Ni) nanoparticles (NPs) by the elec-trophoretic deposition (EPD) method combined with a fumace annealing at 800 ℃ under N2 atmosphere.Modifications of structural and electron field emission (EFE) properties of the metal-doped films are investigated with different metal NPs concentrations.Our results show that the surface characteristics and EFE performances of the samples are first enhanced and then reduced with metal NPs concentration increasing.Both the Ti-doped and Ni-doped nano-diamond composite films exhibit optimal EFE and microstructural performances when the doping quantity is 5 mg.Remarkably enhanced EFE properties with a low tum-on field of 1.38 V/μm and a high current density of 1.32 mA/cm2 at an applied field of 2.94 V/μm are achieved for Ni-doped nano-diamond films,and are superior to those for Ti-doped ones.The enhancement of the EFE properties for the Ti-doped films results from the formation of the TiC-network after annealing.However,the doping of electron-rich Ni NPs and formation of high conductive graphitic phase are considered to be the factor,which results in marvelous EFE properties for these Ni-doped nano-diamond films.