The incorporation of carbon nanotubes (CNTs) is expected as an effective path for tailoring mechanical properties of polymer blends. In this study, acid-modified multiwalled CNTs (A-MWCNTs) were introduced into polystyrene/maleic anhydride-g-(styrene-ethylene-butadiene-styrene) (PS/SEBS-MA) blends. By altering the mass ratio of PS/SEBS-MA from 80/20 to 60/40, the biphase structure of blend was changed from sea-island-like type to quasi co-continuous structure, of different mechanical behaviors. In 80/20 mass ratio, the impact strength was improved while the tensile strength was unchanged with increasing A-MWCNTs content, whereas a simultaneously toughening and strengthening effect was achieved for the compound with 60/40 mass ratio. Scanning electron microscopy, polarized light microscopy, dynamic mechanical analysis, and rheological measurements were carried out to detect the distribution of A-MWCNTs in the blends. The results demonstrated as increasing the nanotube loading from 0 to 3 wt%, A-MWCNTs might gradually migrate into continuous PS phase in 80/20 mass ratio, due to the low content of SEBS-MA, while they were totally packed in SEBS-MA region within the entire loading range used in 60/40 mass ratio due to its high content of SEBS-MA. This study provides guidance on the design and preparation of high performance ternary polymer/elastomer/inorganic filler composites. POLYM. ENG. SCI., 2012. (C) 2011 Society of Plastics Engineers