Chemical vapor deposition (CVD) diamond is an emerging material in the industry. Polishing CVD diamond film to approach the mirror-like state is necessary in a lot of applications, such as optical lens, missile radome, gyrotron windows, X-ray masks, heat sink, bio-sensors, UV detectors, etc. There are several kinds of machining mechanisms existing during polishing diamond film, and they can mainly be separated into mechanical, chemical assisted and catalytic mechanisms. In the current study, the traditional mechanical polishing and chemical assisted methods are both chosen to remove the tip of CVD diamond crystal. Different grit sizes of diamond paste and strong oxidizing agents are used respectively. From the morphology of SEM, the damages after mechanical polishing and chemical assisted methods are quite different. Damages after chemical assisted polishing are much less than they after mechanical polishing. Some of the morphological evidences only show in chemical assisted polishing method. In addition, the results of the surface roughness after the two kinds of polishing methods are shown in the AFM images. Mechanical method takes longer process time than chemical assisted method to lower CVD diamond roughness. The surface roughness of diamond film is therefore from 1200nm down to 600nm after mechanical polishing; however, the roughness is from 1200nm down to 100nm after chemical assisted mechanical polishing.