Abstract: The Ni-diamond composite coating can be obtained by the electro-deposition with the diamond particles added in electrolyte. To obtain the rapid fabrication method of Ni-diamond composite coating, the electrochemical method was used to investigate the mechanism of Ni-diamond electro-deposition. The effect of Co and P addition on electro-deposition process of Ni-diamond was studied. The results show that, by analyzing the cyclic voltammetry, the electro-deposition potential of Ni-diamond become positive with the addition of Co or Co and P, which means the crystallization drive force of nickel is decreased and the deposition of nickel is improved. By the electrochemical impedance spectroscopy analyzing, the deposition process of cathode presents two time constants. The cathodic deposition reaction is carried out in two steps, and the Co is deposited before the Ni deposition. Then, the transfer resistance of Ni-Co-P is smaller. The deposition rate is enhanced by the Co and P addition.