Multiwalled carbon nanotubes (MWCNTs) were modified with a noncovalent functionalization method via ultrasonication in the presence of poly(sodium 4-styrenesulfonate) (PSS) as a modifier. The PSS-functionalized MWCNTs dispersed well in PCL matrix and showed good interfacial adhesion with PCL after being incorporated by solution coagulation. The isothermal crystallization kinetics of neat PCL and PCL/MWCNT nanocomposites were comparatively investigated by DSC. It was found that the overall crystallization rate of PCL was enhanced significantly, while the crystallization mechanism was unchanged by incorporation of the functionalized MWCNTs. The addition of only 0.1 wt % MWCNT caused 23.4 times improvement in overall crystallization rates, ascribing to the efficient nucleating effect of well-dispersed PSS-functionalized MWCNTs toward crystallization of PCL in the nanocomposites. The spherulitic morphology observation by polarizing optical microscope confirmed the nucleating effect. X-ray diffraction investigation indicates that the crystal structure of PCL remained unchanged after incorporation of PSS-functionalized MWCNTs.