A 5-cm diameter, 1-μm (peak to valley) rough diamond film has been polished to optical quality by an oxygen–ion beam. The rms and peak-to-valley roughness values of the polished surface have been lowered to 5.5 and 55.4 nm, respectively. Reactive ion beam polishing provides a low-temperature (140°C), noncontact, and precisely controllable approach to improving the surface quality of diamond films. This progress is essential to the use of diamond coatings in optical applications.