ABSTRACT: A realistic rock mass model is extremely important in order to obtain accurate and reliable results from numerical simulations of blocky rock mass deformation and stability. A probability based rock mass geometrical model with a three-dimensional discontinuity network is proposed in the present study. The rock mass geometry and discontinuities in the rock mass model are built based on site investigation data and stochastic analysis of the discontinuity mapping results. The discontinuities and/or sets of them, either finite or infinite in size, planar or non-planar, convex or concave in shape, can be implemented in the three-dimensional rock mass model. Key block analysis and support design are then suggested to the stochastic rock mass models. A few suggestions have also been discussed to achieve a realistic rock mass model in numerical analysis. These includes development of a realistic geological rock mass model, adopting realistic constitutive relations to intact rock and rock joints, use of realistic numerical model to technically handle numerical difficulties, and achievement of realistic and affordable computational cost, etc.