Polyvinylchloride (PVC)/poly(acrylonitrile-styrene-acrylate) (ASA)/multi-walled carbon nanotubes (MWCNTs) nanocomposites were prepared. The plasticizing behavior, dynamic mechanical properties, mechanical properties and thermal stability of the nanocomposites were studied. The results demonstrate that the plasticizing time shortens as the MWCNTs content increases. The nanocomposites show the best impact strength, which is 83.7 % higher than pure PVC/ASA blend and is 2.1 times higher than pure PVC, when the MWCNTs content is 0.054 wt%. MWCNTs can enhance the thermal stability of PVC/ASA blends; the initial decomposition temperature (T (id)) and thermal degradation activation energy (E (a)) increase by 13.1 A degrees C and 5.7 kJ mol(-1), respectively, when the MWCNTs content is 0.066 wt%. The storage modulus (E (')) and glass transition temperature (T (g)) also increase when MWCNTs are added. MWCNTs can be used as an efficient toughening modifier and processing aid for PVC/ASA blends.