The work aims to obtain low cost and high efficiency single crystal diamond polishing process by studying the influence of nickel and cobalt metal powders with catalytic activity on the mechanical polishing of single crystal diamond. IIa single crystal diamond prepared by high temperature and high pressure method (HPHT) was used as sample. The (100) plane of single crystal diamond was polished mechanically along [100] direction. The polishing media were diamond abrasive paste, diamond powder, diamond and nickel mixed powder, and diamond and cobalt mixed powder. The surface roughness of the sample was tested by atomic force microscopy (AFM). The surface morphology and elemental composition of the samples were characterized by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). When diamond powder was used as grinding powder, the polishing rate was the highest, reaching 900 mm/h, but the surface roughness was relatively poor, 4.15 nm. When nickel powder or cobalt powder mixed with diamond powder was used as polishing medium, single crystal diamond could be polished efficiently. Among them, the best polishing medium was cobalt and diamond mixed powder. The polishing rate was 875 μm/h, and the surface roughness was 1.52 nm. From the experiment results, the high efficiency and high quality polishing of single crystal diamond can be achieved by mixing nickel or cobalt metal powder with diamond powder as polishing medium.