Indentation tests and single-point scratch tests are probably the simplest methods of measuring the elastic, plastic and fracture behavior of brittle materials. In this paper, the nearsurface mechanical properties of KDP single crystal have been investigated including the elasticity like Young’s modulus E, and the plasticity like the hardness H. These material properties can be used to predict the material responses in optical manufacturing operations. Hardness and elastic modulus on different crystal plane of KDP single crystal have been examined under different loads by nanoindentation test, and the influence of the indentation load on hardness and elastic modulus have been also analyzed systematically. The results show the nanoindentation size effect, that is, the hardness and elastic modulus increase as the indentation load decreases. The hardness and elastic modulus have strong anisotropy in the different crystallographic orientation of the same crystal plane.