Nanocomposites of poly(l-lactic acid) (PLA) with fumed silica nanoparticles (SiO2), montmorillonite (MMT) and oxidized multi-walled carbon nanotubes (o-MWCNTs), containing 2.5wt% nanoparticles were prepared, by solved evaporation method. SEM micrographs evidenced fine dispersion of the nanoparticles into PLA matrix. This has as result to act as efficient reinforcing agents increasing the storage modulus, as was verified from DMA analysis. The nanoparticles were found to be effective nucleaging agents in cases of silica nanoparticles and MWCNT. On cooling from the melt crystallization was accelerated by the presence of nanoparticles and the effective activation energy calculated using the isoconversional method of Friedmann decreased. The nucleation activity was calculated. Cold-crystallization was also affected by the presence of nanoparticles. However, it seems that the phenomenon begins at lower temperatures and this results in formation of imperfect crystalline structure which reduce macromolecular chain mobility of the remaining amorphous polymer, finally limiting the ultimate crystallinity.