Abstract: An experimental study on the mechanisms of deformation and failure of NaCl single crystals and salt-lake rocksalt was carried out by means of uniaxial compression tests and optical microscopic observations. There is a convenient condition for this study since the salt-lake rocksalt is of simple characters such as the low strength and low stiffness, the simple structure and texture, the large grain-scale and the transparent crystals of NaCl in which the stress-induced birefringence streaks appears under the crossed polarized light, etc. It is shown in the experiments that the weakness plane effect plays an important part in the procssses of deformation and failure of salt-lake rocksalt.Based on the experiments, a discretization method of nonlinear constitutive relationship of rocks was presented in this paper in stead of the phenomenological methods which approximate constitutive equations to the apparent stress-strain curves. The main points of this method include: （1） developing a probabilistic finite element model which consists of solid elements and weakness plane elementsj （2）inver-sion of the mechanical behaviors of weakness plane by Newton's method; （3）mathe-matical simulation of complete process of deformation and failure of salt-lake rocksalt by the finite element method.