The toughness of poly(styrene-block-butadiene) star block copolymer/polystyrene (PS) blends have been investigated using the essential-work-of-fracture approach. The blends show a co-continuous or layer-like structure of polystyrene-rich and polybutadiene-rich domains arising from the used extrusion process. A tough-to-brittle transition at a critical domain size of polystyrene-rich domains of about 50nm and a maximum in the non-essential work of fracture at 20-30% PS (co-continuous morphology) have been found.