Material toughening due to crack surface friction under mixed mode loading is studied. A mechanistic model is developed for a crack with an intergranular fracture surface under mixed Mode I/Mode III loading. The model takes into account several microstructural and material parameters such as grain shape and friction coefficient between grain boundaries. By employing a self-consistent, fracture mechanics approach, the governing integral equation with a singular kernel is obtained. A special numerical technique is used to solve the integral equation. The frictional stress distribution along the crack face is obtained for different grain shapes and frictional coefficients. The results show that, because of crack surface friction, the material toughens significantly. The toughening ratio increases rapidly with the relative magnitude of the Mode III component of the applied load. Moreover, two parameters, the oblique grain boundary angle and the friction coefficient, are identified as the controlling parameters for the toughening behavior. Using an energy-based crack tip fracture criterion, fracture loci in KI - KIII space are constructed to serve as a failure criterion for mixed mode fracture. The model predictions are also compared with experimental measurements. Good agreement is obtained. Potential applications of the model to surface crack growth under rolling contact loading in the presence of lubricating fluids are discussed.