This paper focuses on the process of electrorheological (ER) finishing optical surfaces. Experiments on K9 mirrors were conducted. In one experiment, the operating distance was varied over 0.5–0.8 mm with the voltage at 2000 V. The maximum peak-to-valley (PV) reduction was obtained at the distance of 0.5 mm, where the PV value was reduced from 58.71 to 25.03 nm. In another experiment, the voltage was varied over 1500– 3000 V with operating distance at 0.5 mm. The final surface roughness (Ra) achieved was as low as 2.5 nm. A higher voltage produced a higher relative reduction of the Ra. These experimental results validated the process.