Numerical modeling, using two-dimensional distinct element methods, is used to examine the effect of stress on the stability of a fractured rock mass. The critical stress state depends on the differential stress, mean stress, and fluid pressure and is represented by a surface which bounds all stable stress states. By examining the critical stress states under different loading conditions it is possible to define the stability/instability in terms of the far-field differential stress and effective mean stress. Thus the strength of fractured rock can be represented by a macroscopic frictional component (μz) and a cohesion (Cz), which differ from the corresponding parameters for individual fractures. A series of simulations are used to examine the effects of fracture network geometry, such as fracture density, fracture length, and fracture network anisotropy, on the instability strength. A steady decrease in equivalent frictional strength (μe) with increasing fracture density was found. For the same fracture density, rock masses with fewer, but larger, fractures had lower instability strength. As networks became more anisotropic, the orientation of the fractures in relation to the loading direction had a considerable impact on the instability strength and deformation pattern. The effects of loading direction in relation to fracture set orientation have been examined for two fracture networks with different anisotropy coefficients. Where the directions of the principal stresses were parallel to the fracture sets, extensional deformation was observed. Otherwise, dilational shear deformation modes develop, within which sliding, opening, and block rotation occur. ¿ 2001 American Geophysical Union