Chemical vapor deposited (CVD) diamond film is a good materials for cutting tools as its a series of excellent properties. But because of its polycrystalline morphology, CVD diamond thick film has a rough surface that limits its application in engineering. In this paper, study was carried out on the mechanical lapping of diamond film. It is shown that surface roughness of the film was reduced from Ra 4.5μm to Ra 0.2μm after 50-minute polishing. The surface integrity of polished diamond thick film was investigated, which includes surface roughness, morphology and residual stress. There are a lot of micro defects such as grooves, gas cavities and micro cracks on the polished surface, which are the intrinsic defects generated in the deposition process of CVD diamond film. The tensile stress of the film reduced through polishing as the release of the deformation energy stored in the film.