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 used 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. In the definition of rock damage yield function, many authors considered only the tensile microcracking. Since quasi brittle materials such as rock degrade under tensile and shear microcracking, separate positive and negative damage yield functions were introduced. The proposed damage yield functions are formulated in the framework of a damage model which was coded in C++ environment and implemented into a commercial code. Accordingly, the proposed model was applied to the simulation of brittle rocks behavior. The three point load bending test of a brittle rock was simulated numerically and numerical findings were presented. The analysis results show a very good match between numerical and experimental observation especially in the post-elastic region.