In the present work, the crystalline structures and the melting behaviors of poly(L-lactide) (PLLA) obtained after being annealed at different conditions have been investigated through differential scanning calorimetry and wide-angle X-ray diffraction, respectively. To improve the crystallization of PLLA, functionalized. multiwalled carbon nanotubes (f-MWCNTs) are introduced into PLLA. Our results show that by prolonging the annealing duration or enhancing the annealing temperature, the degree of crystallinity of PLLA gradually increases. Very important, the addition of f-MWCNTs promotes the cold-crystallization of PLLA dramatically even at relatively lower annealing temperature or in shorter annealing duration. Further results show that, whether in neat PLLA or in PLLA/f-MWCNTs nanocomposite, only a form crystal forms during the annealing process. The glass transition temperature shifts to high temperatures because of the increase of crystallinity. F-MWCNTs exhibit great heterogeneous nucleation effect for PLLA crystallization through enhancing the nucleation density, leading to homogeneous and tiny spherulites formation in a very short time. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 326-339, 2009