The nanocrystalline diamond (NCD) films are implanted by oxygen ions with a dose of 1×1012 cm-2 and subsequently annealed at 700, 800, 900 and 1000 ℃, respectively. The microstructure and electrochemical properties of these NCD films are investigated systematically and the results show that the potential windows of the unannealed sample (O120) and 1000 ℃ annealed sample (O121000) increase up to 4.6 V and 3.61 V, respectively. The mass transfer efficiencies of the two samples are also better, indicating that the oxygen ion implantation and 1000 ℃ annealing can improve the mass transfer efficiency of NCD film. The results of infrared spectrum measurements show that there are no hydrogen atoms that are terminated to the surfaces of samples O120 and O121000, while hydrogen atoms terminate to the surfaces of the other samples. It is indicated that oxygen ion implantation and 1000 ℃ annealing can damage hydrogen terminations in the surface, which improves the electrochemical performances of NCD films. Raman spectrum measurements suggest that high content of diamond phase, small internal stress and more disordered amorphous carbon can improve the electrochemical properties of NCD films. When the number or size of sp2 carbon clusters in amorphous carbon grain boundaries decreases, the electrochemical properties of NCD films become better.