To provide a novel technique to prolong the service life of diamond cutting tools, a thermo-chemical erosion method by using nano-scale copper oxide under vacuum atmosphere was proposed to perform the post-treatment of natural diamond crystals, which had been pre-polished with mechanical lapping procedure. As indicated in the experiments, when the temperature under vacuum atmosphere was 100 ℃, the oxidation and deoxidation appeared between the damaged diamond carbon atoms on the topmost surface layer and the copper oxide powder. The surface roughness of natural diamond crystal declined slowly with the increment of erosion time, i.e. generating a weak planarization tendency. When the vacuum atmosphere temperature was 200 ℃, two hours erosion time was considered as the optimal configuration. At 30 nm indentation depth, the micro-hardness and elastic ratio of the eroded diamond crystal surface reached the maximal value. If the erosion time was larger than two hours, the micro-hardness and elastic ratio declined. 