Differential scanning calorimetry (DSC), polarized optical microscopy, and X-ray diffraction methods were used to investigate the isothermal crystallization behavior and crystalline structure of poly (epsilon-caprolactone) (PCL)/multiwalled carbon nanotube (MWNT) composites. PCL/MWNT composites were prepared via the mixing of a PCL polymer solution with carboxylic groups containing multiwalled carbon nanotubes (c-MWNTs). Both Raman and Fourier transform infrared spectra indicated that carboxylic acid groups formed at both ends and on the sidewalls of the MWNTs. A transmission electron microscopy micrograph showed that c-MWNTs were well separated and uniformly distributed in the PCL matrix. DSC isothermal results revealed that introducing c-MWNTs into the PCL structure caused strongly heterogeneous nucleation induced by a change in the crystal growth process. The activation energy of PCL drastically decreased with the presence of 0.25 wt% c-MWNT in PCL/c-MWNT composites and then increased with increasing MWNT content. The result indicated that the addition of c-MWNT to PCL induced heterogeneous nucleation (lower total activation energy) at a lower c-MWNT content and then reduced the transportation ability of polymer chains during crystallization processes at a higher MWNT content (higher total activation energy). A correlation between the crystallization kinetics, melting behavior, and crystalline structure of PCL/c-MWNT composites was also discussed. (c) 2005 Wiley Periodicals, Inc.