A micromechanical constitutive model, based on the responses of sliding cracks and elliptic cracks, is used to simulate the inelastic deformation of brittle rock. In addition to the usual incremental equations, the model has the following features: (1) activation equation that include the effects of friction and relate the far-field stresses of sliding and elliptic crack openings and lengths, (2) a stress intensity analysis of the sliding cracks that determines how much the cracks may grow, and (3) expressions for the crack strains that are constructed directly from the amount of crack sliding, opening, growth and the number of sliding and elliptic cracks in a given region. Numerical simulations of uniaxial and triaxial loading experiments on westerly granite were compared with experimental data to determine the validity of the constitutive model. These comparisons show that most of the uniaxial stress data and the loading portion of the triaxial data can be simulated quantitatively. The unloading portion of the triaxial data cannot be simulated properly, suggesting that a nonfrictional mechanism, not included in our model, may dominate the response in this region.