Diamond film is widely used in mechanical,electronic,optical Industries due to its excellent physical and chemical properties of the highest hardness, low friction coefficient, high elastic modulus and good antiwear and anticorruptibility. With the widing of its application, high efficiency polishing of diamond film is a very difficult problem need to be solved. In this thesis, the technique and the mechanism of the chemical mechanical polishing will be further investigated throughly.A equipment which parameters can be easily justified was designed to study the effects of prssure, temperature, oxidant, polishing speed, additive on the polishing for the technique and the mechanism of polishing.The oxidation in a certain temperature and oxidant were studied. It was found that diamond film in the oxidant was oxidated when temperature reaches 500℃ while the single crystal diamond oxidated markedly when temperature reaches 600℃.The chemical mechanical polishing of single crystal diamond on the designed equipment indicates that temperature and pressure play the main roles in the polishing with observing and testing the sample polished in series. The roughness Ra of the polished diamond surface got to 1 66nm under AFM with the condition of temperature 400℃ and pressure l.OMPa. It can be inferred that chemical reactions occurred in the process of polishing from the testing of the sample polished.It was also found that temperature and pressure play critical roles in polishing of diamond films. The polishing rate can be enhanced at least 50% while adding suitable amount of SiO_2 to the oxidant. But it made the polishing bad while adding CrO_3. The polishing rate and quality would be better if temperature gets higher,rotating speed faster, pressure bigger, adding SiO_2, mechanical treatment as a pretreatment. The roughenss Ra of polished surface can get to 1.66nm. Chemical reaction also occurred in the course of polishing, and the polishing is a combination of chemical polishing, chemical eroding, diffusing of carbon, transformation of diamond into amorphous carbon and graphite.The best polishing technique may be temperature 400℃, pressure 1.2MPa, rotating speed 90rpm, KNO_3 and LiNO_3, adding 30% amount of SiO_2.