Poly(ethylene terephthalate) (PET) nanocomposites were prepared by melt-extruding mixtures of PET and functionalized multiwalled carbon nanotubes (MWNTs) with some interaction with PET molecules. For the functionalization of MVv'NTs, benzyl isocyanate and phenyl isocyanate with different molecular flexibility were employed on the surface of the MWNTs via chemical modification, respectively. The reaction for functionalization of MWNTs was confirmed by FTIR and transmission electron microscopy (TEM) measurements. TEM observations indicated that both benzyl and phenyl isocyanate groups covered the surface of the MWNTs after functionalization. The PET nanocomposites containing isocyanate groups showed improved mechanical properties, including the tensile strength and tensile modulus, compared with those with pristine and acid-treated nanotubes. These improvements were ascribed to pi-pi interactions between the aromatic rings of PET molecules and the isocyanate group in MWNTs. The functionalized MWNTs showed a better dispersion of carbon nanotubes in the matrix polymer and a different fractured cross-section morphology in scanning electron microscope measurements relative to the pristine MWNTs. The crystallinity of the functionalized MWNT-PET nanocomposites was significantly higher than that of the pristine and acid-treated MWNTs. FTIR results indicated that the presence of carbon nanotubes induced trans-conformation of PET chains, and trans conformation was particularly dominant in PET composites incorporating MWNT-phenyl. (c) 2008 Wiley Periodicals, Inc.