Force-deformation relationships of rock joints are important in the study of mechanical behavior of fractured or jointed rocks. Experimental studies have shown that rock joints often exhibit deformation hardening and anisotropic behavior under shearing loads. This paper focuses upon the mathematical modeling of force-deformation behavior of anisotropic rock joints by explicitly considering interaction of asperities on a joint surface. Elastic deformations and inelastic frictional sliding are considered at inclined asperity contacts. A modified spherical harmonic expansion is introduced to model the orientation distribution of asperity contacts. The resultant joint force-deformation model is used to obtain the normal and shear stiffness behavior of anisotropic joints. Results obtained from the model show that the asperity contact orientation distribution is essential for representing the anisotropic nature of interface geometry and for modeling deformation hardening of interfaces. The model results are compared with experimental data culled from the literature. ¿ 1999 American Geophysical Union