In a previous study, two triblock copolymers of styrene/ethylene-butylene/styrene (SEBS), of different molecular weights, were used to compatibilize a blend of 80 vol% polystyrene (PS) and 20% ethylene-propylene rubber (EPR). The emulsification curve, which relates the average minor phase particle diameter to the concentration of interfacial agent added, was used to quantify the effect of the interfacial agents on the blend morphology. Links between morphology, interface, and properties were established by combining the emulsification curve with a fracture mechanics approach. The aim of this work is to foster the understanding of the effects of these two triblock copolymers on the fracture behavior of the blend over various loading rates and temperatures. The focus is on the brittle-ductile transition in fracture behavior, which is a critical condition for the application of these materials. It has been found that adding an interfacial agent lowers the temperature at brittle-ductile transition. However, this effect is much more pronounced for the copolymer with a lower molecular weight. The time-temperature dependence of fracture performance of the blend is also affected by the interface and morphology. When loading rate increases, the shift of the temperature at brittle-ductile transition is less significant for the blend with an interfacial agent having a lower molecular weight. The effect of loading rate and temperature on the brittle-ductile transition in fracture performance of the blends is controlled by an energy activation process. Adding the interfacial agents results in a plasticizing effect of the polystyrene matrix and a reduction in the energy barrier controlling the fracture process. With the addition of interfacial agent, the yield stress slightly increases at low concentration, attains a maximum value, and then decreases. The increase in yield stress confirms the coupling role of the copolymer and is in agreement with the observed emulsification curves. The reduction of yield stress and increase in ultimate strain with the copolymer concentration demonstrate the plasticizing effect of the interfacial agent. The result of stress relaxation tests also confirms the above effects of the interfacial agent.