We have manufactured polylactide (PLA) nanocomposites reinforced with layered silicates by the melt compounding method and have investigated their structure, thermal stability under oxygen or nitrogen atmosphere, and temperature-dependent mechanical modulus by adopting XRD, SEM, TGA, and DMA. Octadecylammonium-modified montmorillonite (organo-MMT) is used as an organically modified layered silicate and, for comparison, sodium montmorillonite (Na-MMT) is adopted as a purely inorganic layered silicate. X-ray diffraction patterns and SEM images confirm that the organo-MMT is intercalated and dispersed homogenously in the PLA matrix, whereas Na-MMT remains unchanged by forming agglomerated domains in the matrix. Unlike the PLA/Na-MMT nanocomposites, the thermal stability of PLA/organo-MMT nanocomposites is found to be substantially improved with the increment of organo-MMT content, especially under oxygen gas atmosphere. In addition, the mechanical modulus of PLA/organo-MMT nanocomposites in the glassy state are highly improved.