Poly(epsilon-caprolactone)diol (PCL) with five different molecular weights (e.g., 400, 2000, 3000, 4000, and 8000 g/mol) was grafted onto the surface of functionalized-multiwalled carbon nanotubes (f-MWNTs) by grafting to approach to form PCL-grafted-MWNTs (PCL-g-MWNTs). The grafting was confirmed by Fourier transform infrared spectroscopy and transmission electron microscopy (TEM). Thermal stability of PCL-g-MWNTs improved with the addition of f-MWNTs in PCL, and the maximum was observed at 6 wt %, of f-MWNTs. Differential scanning calorimetric (DSC) studies showed that crystallization temperature, melting temperature, heat of crystallization, and heat of fusion decreased with the weight proportion of f-MWNTs in the composites, but they showed an increasing trend with increasing molecular weight of PCL in the composites. DSC results were of good agreement with the X-ray diffraction data, as crystal-linity decreased with the incorporation of f-MWNTs in PCL. The crystallite size (P-hkl) of PCL in (110) direction increased, but in (200) direction it decreased. The size anisotropy (P-110/P-200) of the crystals in (110) and (200) directions increased regularly, indicating that the crystal growth was nonuniform in the two directions. The morphological observation by scanning electron microscopy and TEM showed that the PCL-g-MWNTs had higher diameter than raw multi-walled carbon nanotubes. Polarized optical microscopy showed that PCL-g-MWNTs had core/shell structure with the nanotubes as the 'hard' core and the hairy polymer layer as the 'soft' shell. (C) 2008 Wiley Periodicals, Inc.