In a previous article, we reported the preparation and characterization of a nanocomposite of poly(L-lactic acid) (PLLA) and silica via the in situ melt polymerization of L-lactic acid in the presence of acidic silica sol. In this study, the isothermal crystallization and melting behaviors of a PLLA/silicon dioxide (SiO2) nanocomposite with 5 wt % well-dispersed SiO2 nanoparticles (PLLASN5) and pure PLLA were comparatively studied with differential scanning calorimetry and polarized optical microscopy. The SiO2 nanoparticles acted as nucleation agents in the PLLA matrix and enhanced its nucleation rate and overall crystallization rate, especially at high crystallization temperatures. However, no deleterious effect on the crystal morphology or crystallinity was observed. The crystals that formed at a low temperature were imperfect; therefore, double melting peaks occurred during the second heating scan because of melt recrystallization. With the crystallization temperature increasing, the crystals became increasingly perfect; as a result, the low melting peak increased and shifted to a higher temperature. The existence of SiO2 nanoparticles had no effect on the equilibrium temperature of the PLLA matrix. Pure PLLA and PLLASN5 have the same equilibrium temperature of 171.5 degrees C. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 1045-1050, 2009