The subsurface damage of Potassium Dihydrogen Phosphate (KDP) crystal machined from line-incision to grinding and polishing was detected using section experiment and step etching experiment. The OLYMPUS MX40 optic microscope was used to observe the surface etching phenomena and the sub-surface micro-crack configurations

and to measure the micro-crack depth. The results show that the subsurface micro-crack configurations induced by line-incision is mainly of "bias-figure"

and the max micro-crack depth is 85.59 μm. While by the #600 grinding wheel the max depth reduces to 8.55 μm. The square dislocations etching pits with high density are on (001) crystal surface and the low density etching pits with the figure resembled trapezium are on the tripler surface. This study affords a excellent detection and analysis method for the subsurface damage of KDP crystal machining.