Isothermal and nonisothermal crystallization kinetics of poly(l-lactide) and its nanocomposites with different multiwalled carbon nanotubes (i.e., pristine and functionalized) were investigated. The effects of chain extension and nanofiller content on the crystallization kinetics were also explored. The Avrami analysis was applied as a useful tool for the study of isothermal crystallization kinetics, which was also supported by optical microscopy observations. In addition, in nonisothermal crystallization process, the Avrami and Mo methodologies were used to study the crystallization kinetics as well as the evaluation of activation energy from isoconversional methods. The activation energy was always negative for the process driven by the secondary nucleation. Great differences were found between the studied samples, but activation energy values indicated a favored crystallization (i.e., less negative energies) when functionalized multiwalled carbon nanotubes were added at a concentration of 0.2¿wt%. Cold crystallization was also influenced by the multiwalled carbon nanotube content due to the nucleating effect and the existence of geometrical constrictions that limited the size of crystalline domains. Small-angle X-ray scattering studies showed that the size of crystal lamellae after cold crystallization could vary by a magnitude of 3¿nm depending on the way as the sample was previously cooled (i.e., by the presence of incipient crystalline domains).