Poly(styrene-block-butadiene-block-styrene) triblock copolymer (KR05) was melt mixed with methyl methacrylate-styrene copolymer (MS-200) in an internal mixer, and the resultant blends were compression moulded to examine the morphology and the mechanical properties. KR05 in KR05/MS-200 blends incorporated in an internal mixer exhibits an alternate lamellar structure between the PB and the PS phases, and phase separation from MS-200 as reported previously for the twin-screw extruded blends. Internal mixing should be a powerful tool for melt mixing of KROS with MS-200, since KR05/MS-200 blends after internal mixing could roughly parallel those after partially intermeshing counter-rotating twin-screw extrusion in degree of mixing. Ductile KR05 is toughened by incorporation of brittle MS-200 and synergistic improvement of the impact strength is observed for compression mouldings of the blends with the composition between 10 and 30 wt% MS-200. When the composition of KR05/MS-200 blends was varied, MS-200 domain morphology appropriate for toughening the blends was examined. In this case KR05/MS-200 blends are remarkably toughened with the morphology that spherical MS-200 domains ranging from 0.3 to 0.5 mu m in actual diameter are densely dispersed in orderly lamellae of the KR05 matrix.