Ion beam figuring (IBF) is introduced to overcome the disadvantages of conventional polishing processes in optical machining. The principle and method of IBF are introduced, and the key techniques required to implement an IBF process are also discussed. To demonstrate an IBF process, a 98 mm planar ceramic glass was processed on an IBF system. Its surface figure accuracy root-mean-square (RMS) is improved from initial 0.136λ to final 0.010λ(λ=632.8 nm) through two IBF processes, and the mean iterative convergence ratio is 3.7. The result shows that the IBF is a deterministic process, which results in a high surface figure accuracy without edge effect and a corresponding high convergence ratio of surface figure. Due to the unique characteristic of the sputtering process in IBF, machining an aspheric surface is no more difficult than machining a planar surface.