Chemical vapor deposition (CVD) diamond thin films are widely used in modern industries. However, due to the nature of polycrystalline, thin films are required to be polished in the final process to increase its surface quality. Thermal-chemical polishing is known for its less processing time and low cost. In this paper, the experiments are carried out to observe the effect of processing conditions such as temperature, rotational speed, polishing pressure, and substrate material on the surface roughness and on the material removal rate of the chemical vapor deposition diamond (CVDD). At the same time, the processing mechanism for this thermal-chemical polishing is investigated, and a polishing model is built for comparison with the experiment results. The results show that the material removal rate is affected mainly by the diffusing rate in the Fe-C polishing system. By using the model, the approximate value of material removal rate can be calculated according to the polishing temperature.