Two melting peaks are generally observed in a heating scan for isothermally crystallized poly(lactic acid) (PLA)/carbon black (CB) and PLA/modified carbon black (MCB) composites. To investigate the origin of the above double melting behavior, the melting behavior after isothermal crystallization was analyzed with differential scanning calorimetry, wide-angle X-ray diffraction, and small angel X-ray scattering techniques. The double melting of the crystallized samples can be explained by the model of two populations of lamellae, the double peaks of low and high temperatures are contributed to the melting of the small lamellae produced by secondary crystallization and that of the major crystals formed in the primary crystallization process, respectively. Spherulitic growth rates of the neat PLA and PLA/MCB composite were analyzed and the occurrence of a regime transition was demonstrated. For the PLA, a clear regime transition was observed at around 125 degrees C. For the PLA/MCB, it occurred at 130 degrees C. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47:1971-1980, 2009