Polished diamond coated silicon carbide can be a potential candidate material for making optical mirrors, due to the excellent properties. At present, five typical types of diamond films are deposited on RB-SiC substrates by hot filament chemical vapor deposition, and then polished by mechanical polishing. It is found that the boron-doped micro-crystalline and undoped nano-crystalline composite diamond (BDMC-UNCCD) coated specimen performs the best before, during, and after polishing. The film surface composed of nano-sized diamond grains has relatively low surface roughness and hardness, which are beneficial for the efficient polishing, and under the present condition only such a surface can be completely polished to a homogeneous mirror surface. The micro-sized diamond grains and the boron incorporation in the underlying BDMCD layer can enhance the film-substrate adhesion, which plays an important role in the film integrity during the polishing or subsequent applications. In conclusion, the polished BDMC-UNCCD coated RB-SiC specimen indeed shows low surface roughness (Ra=5.41 nm), high hardness (71.47 GPa), high elastic modulus (746 GPa), favorable surface shape accuracy (RMS=0.083λ), and considerable reflectivity in the short-wavelength range.