Surface quality is an important issue for KDP crystal implemented in high power lasers. Single Point Diamond Turning is the only way to get the required transmitted wavefront and damage threshold, specially in the UV band of the laser Megajoule. The aim of the process development was to reach an rms roughness lower than 3 nm using a vertical fly-cutter, an appropriate cleaning equipment and an interferometric microscope for measurements. A Taguchi Design of Experiment method has been sued to optimize the fly-cutting process parameters on 100 X 100 mm2 KDP crystals.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.