CVD diamond coated tungsten carbide tools have been used for cutting and drilling of soft materials such as aluminum and copper alloys. However, it is very difficult to obtain a tool having a sharp tip of the order of sub-μm by mechanical abrasive polishing methods. Therefore, we applied ion beam processing for sharpening the cutting edge of diamond coated tungsten carbide tools. Result shows that it is possible to obtain a 20–80 nm order tip width of a CVD diamond coated knife processed by a 0.5–10 keV Ar + ion beam, and the sharpening speed of a tip of the knife depends on the ion beam energy. Namely, a tip width of a knife processes by a 1.0 keV Ar + ion beam at an ion dose of 2.7 × 10 20 ions/cm 2 becomes 20 nm, and a tip width of a knife processed by a 10 keV Ar + ion beam at an ion dose of 5.4 × 10 19 ions/cm 2 becomes 40 nm. However, a facet with an apex angle in the range of 60–100° was formed on the cutting edge of a knife with an initial apex angle of 55°, and we found that the facet angle can be controlled by choosing ion beam energy of 0.5–10 keV. Moreover, results show that the processed knife machined by a 0.5 keV Ar + ion beam has very smooth rake and flank faces, and also has a small line edge roughness of the cutting edge compared to those of the sharpened knife by a 1.0–10 keV Ar + ion beam.