Matrix-polymer-functionalized multiwalled carbon nanotubes (MWCNTs) are demonstrated as a highly efficient toughening agent for matrix polymers. With poly(vinylidene fluoride) (PVDF) as the matrix polymer, the PVDF/MWCNT-PVDF nanocomposite films show high toughness. With a small load amount of MWCNT-PVDF (0.07 wt %), the nanocomposite film shows a yield point and a constant-stress extension region in stressstrain tests, compared with the typical low-extensibility feature of neat PVDF film. The PVDF/MWCNT-PVDF-0.7 film exhibits a 180-fold increase of toughness and about 38-fold increase in strain at break compared with neat PVDF film. This toughening effect is attributed to (a) homogeneous dispersion of MWCNT-PVDF in PVDF, (b) the high efficiency of load-transfer across MWCNT/PVDF interface, and (c) the long length of the MWCNTs. (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012