AbstractFilms based on a commercial block copolymer and multiwall carbon nanotubes (MWCNTs) were prepared by solution casting. The block copolymer has a star asymmetric chain architecture, with polystyrene chains of different lengths and a polybutadiene core. The block copolymer has a lamellar morphology. The MWCNT were grafted with polystyrene chains under atom transfer radical polymerization (ATRP) conditions. The nanocomposites were characterized by transmission electron microscopy (TEM) and simultaneous small angle X-ray scattering (SAXS)/strain–stress tests. The morphology of the nanocomposites is influenced by the polystyrene chain grafted from the MWCNT. In the case of the nanocomposite based on pristine MWCNT, most of the nanotubes intersect the lamellar domains of the block copolymer. With the increase of the molecular weight and the grafting chain density of the polystyrene anchored to the MWCNT, a favorable interaction between the polystyrene microdomain phases in the block copolymer and the MWCNT is observed. In the case where the MWCNT grafted with the relative highest molecular weight (20 kg·mol−1) and polystyrene grafted content (> 90 wt%), the MWCNT locally templates the lamellar morphology of the block copolymer. The effect on the block copolymer is analyzed based on the orientation of the morphology under an external deformation.