Poly(L-lactide) (PLLA)/multiwall carbon nanotube (MWNT) composites were prepared by the solvent-ultrasonic-casting method. Only very low concentrations of MWNTs (<0.08 wt %) were added in the composites. Isothermal and nonisothermal crystalline measurements were carried out on PLLA/MWNT composites to investigate the effect of MWNTs on PLLA crystalline behavior. DSC results showed that the incorporation of MWNTs significantly shortened the crystalline induction time and increased the final crystallinity of the composite, which indicated MWNTs have a strong nucleation effect on PLLA even at very low concentrations. The nonisothermal crystallization measurements showed that the MWNTs greatly speed up crystallization during cooling. From isothermal crystallization results, both PLLA and PLLA/MWNT composites samples closely followed a relationship based on Lauritzen-Hoffman theory, with the regime II to III transition shifting to lower temperature with increasing MWNT concentration. A double melting peak appeared in both nonisothermal and isothermal measurements. The conditions under which it appeared were found to closely depend on the regime II-III transition temperature obtained from Lauritzen-Hoffman theory. From the magnitude and position of melting peaks, it is proposed that the double melting peak is caused by a disorder-order crystal phase transition. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2341-2352, 2009