CVD diamond film is widely used in mechanical, electronic, optical industry due to its excellent physical and mechanical properties. But highly efficient polishing of diamond film is a very difficult problem to be solved. In this thesis, it is the first time that EDM with graphite electrode is introduced to polish diamond film. And the techniques and the mechanism of the EDM polishing, mechanical polishing, and oxidative-chemical polishing of diamond film were investigated.In EDM polishing with graphite electrode, it was found that the roughness Ra can decrease 2.94um in only two minutes. So polishing diamond film by EDM with graphite electrode is proved effective. The EDM polishing mechanism mainly includes vaporization, microcrack, fracture, and graphitization of diamond film.In mechanical polishing with diamond discs, the results from single factor experiments show that the increase of rotating speed and pressure and the use of coarser diamond disc are helpful to get a better polishing. The orthogonal experiment method was used to optimize the technical parameters and the mechanism of mechanical polishing is advanced to be the micro-cutting. It"s the first time that the transformation of diamond into amorphous carbon is observed in mechanical polishing. Using optimized polishing parameters, an ultra-smooth surface with roughness of Ra 6.93nm is obtained.Oxidative chemical polishing of diamond film is studied on a modified machine. The polishing results indicate that cast-iron plate, higher pressure and higher rotating speed are useful to get a better surface roughness. One kind of mixed nitrate is used as oxidant to decreases not only melting point, but also the cost of experiment, and also to improve the polishing effect, with the result of a diamond film with final roughness of Ra 0.4um. It is first time to find the amorphous carbon and graphite were transformed from diamond in oxidative chemical polishing.