A process based on chemical mechanical polishing has been proposed to polishing ultra-precision nonlinear optical crystal flats with high surface quality. An annular polyurethane pad was employed in the process. An excellent flatness of the annular polishing pad can be obtained using a special conditioner. The newly developed septum and holder system in the process has significantly reduced the rigid punch effect and workpiece/pad deflection brought by the spindle and carrier system in the common chemical mechanical polishing (CMP) process. In addition, the pre-strain of the pad by the septum can effectively eliminate the elastic response of the pad on the exterior of the workpiece and hence greatly reduce the stress concentration on the edge. The ACMP process is especially suitable for the final polishing of fragile crystals that have anisotropic properties and demand strict requirements on surface figure and defects. Crown Copyright (C) 2012 Published by Elsevier B. V. All rights reserved.