Numerical simulations are performed in order to gain a better understanding of the effects of various microstructural features and toughening mechanisms in amorphous PC/ABS polymer blends. Crack tip loading under global small-scale yielding conditions is considered with the blend microstructure explicitly resolved in the near-tip process zone. Constitutive models are employed which account for large visco-plastic deformations, the characteristic softeningrehardening behavior of glassy polymers, as well as the effect of plastic dilatancy in the ABS phase due to rubber particle cavitation. The influence of blend composition and morphology on the local stress distribution and the development of the plastic zone at a stationary crack tip are analyzed. Furthermore, crack propagation and the evolution of fracture toughness are studied using different cohesive surface models for failure in the different phases of the blend microstructure.