The excavation of underground openings generally causes damage to the rock in the vicinity of the openings. The dominant causes of irreversible rock deformations are damage process and plastic flow. Most of the existing elastic-plastic models employed in the analysis and design of rock structures only consider the plastic flow and ignore the full damage process. The common plastic models to simulate the rock failure, does not model the rock realistically and often the important issues such as stiffness degradation, softening, and significant differences in rock response under tensile and compressive loadings are ignored. Therefore, developments of realistic damage models are essential in the design process of rock structures. In this paper, the basic concepts of continuum damage mechanics are outlined. In the definition of rock damage yield function, many authors considered only the tensile microcracking (mode I). Since quasi brittle materials such as rock degrade under shear microcracking (mode Π), separate positive and negative damage yield functions are introduced. The proposed damage yield functions are formulated in the framework of a damage model which was coded in C++ environment and implemented into the commercial code UDEC. Accordingly, the proposed model was applied to the simulation of brittle rocks behavior. The uniaxial compressive testing of a brittle rock was simulated numerically and numerical findings were compared against experimental data and analytical solution. The analysis results show a very good match between numerical, experimental data, and analytical solution especially in the post-elastic region.