Abstract: The mechanical properties and morphology of nano-CaCO₃ / SBS or mSBS / PS composites were studied.In a nano-CaCO₃ / mSBS/ PS system, nano-CaCO₃ particles are mostly encap sulated in mSBS. The core-shell st ructure increases the volume f raction of elastomer (mSBS) and enlarges the effect of mSBS on the mechanical propertiesof this system. On the other hand, the concent ration of nano-CaCO₃ particles decreases in PS mat rix, which prevent s nano-CaCO₃ (especially agglomerated nano-CaCO₃ particles) f rom impairing the mechanical properties of thesystem. Compared with SBS system, the costs of the composites decrease with the increase of optimum loading ofnano-CaCO₃ .