Thin films of diamond nanoparticles were prepared by electrophoretic deposition in water dispersion. The voltage applied to the electrode of electrophoretic deposition gave a crucial influence on the film growth of nanoparticles, and a voltage of 3 to 4V was found to be optimum for film formation. It was possible to obtain uniform coverage by the diamond nanoparticles on a substrate surface at an early stage of film growth without coagulation of the particles. By depositing a thin layer of diamond nanoparticles on a surface of aluminum cathode, electron injection from the cathode to tris(8-hydroxyquinolinato)aluminum was notably enhanced. It was suggested that electrophoretically deposited diamond nanoparticles can play a role of electron-injection layer to organic semiconductors.