Abstract: CVD diamond thick films were etched by oxygen cyclotron ion beams and polished by mechanical methods and the effect was investigated. The influences of gas pressure and bias voltage of anode cylinders on the ion parameters were studied by an ion sensitive probe. The etching of CVD diamond films was accomplished under the optimized gas pressure and bias voltage of the anode cylinder. The optimized gas pressure and bias voltage of the anode cylinder for diamond etching are 0.03 Pa and 200 V, respectively. Under these conditions, the ion temperature is 7.38 eV and the ion density is 23.81010 cm-3. The diamond film was etched for 4 h by the oxygen cyclotron ion beam under temperature of the sample holder of 600 ℃ and then polished for 15 min by mechanical lapping. The results show that the diamond surface roughness decreases from 3.525 m to 2.512 m by ion beam etching, and then decreases to 0.517 m by mechanical polishing. The efficiency of mechanical polishing is dramatically improved when the diamond film is etched by oxygen cyclotron ion beams.