The morphology and mechanical properties of the moulded polymer blends of poly(styrene-block-butadiene-block-styrene) triblock copolymer (KR05) with methyl methacrylate-styrene copolymer (MS-200) were investigated. The KR05 phase of the blends exhibits lamellar morphology and phase separation from the MS-200 phase, and mechanical properties of the blends depend on the microstructural orientation and orderliness. Ductile KR05 is toughened by incorporation of brittle MS-200, and synergistic improvement of toughness is observed for the KR05-enriched blends. Under tensile stress excessive stress concentration leading to critical failure of the blends could be restrained effectively by the far-reaching morphological changes with shear yielding of KR05 lamellae, cavitation in the polybutadiene phase and debonding in the KR05/MS-200 interfacial region. The synergistic effect of toughening is much more prominent for the blends with oriented KR05 lamellae and spherical MS-200 domains than for those with crooked KR05 lamellae and wavily elongated MS-200 inclusions, since on impact the former blends could dissipate larger energy through dominant expansion of the polybutadiene lamellar spacing and delocalized debonding at smooth KR05/MS-200 interfaces. The latter blends show enhanced rigidity and heat resistance since the crooked lamellae offer more resistance against the deformation because of the close and complicated networks of the polybutadiene phase tangled with the rigid polystyrene phase, and because of more efficient interconnection of elongated MS-200 domains.