The effects of residual stress on toughening of brittle polycrystalline materials, in the absence of microcracking, were investigated by considering the mode I stress intensity factor reduction at the tip of a stationary crack under combined applied and residual stress loading. Toughness enhancement associated with a number of model singular and non-singular residual stress fields was evaluated. The singular residual stress fields were used to model grain-sized thermal expansion anisotropy due to grain-orientation differences in a polycrystal. The numerical results indicate that residual stress can significantly toughen a stationary crack against initiation. For the same average value of residual stress, toughness enhancement due to singular residual stress fields is more substantial than that due to non-singular residual stress fields. Sample toughness enhancement results are presented for a single-phase polycrystal failing by intergranular fracture.