Three kinds of novel organic montmorillonites (OMMTs) were prepared by reacting the amino polyhedral oligomeric silsesquioxanes (POSSs) with the OMMTs that had already been modified by cationic surfactants. The layer spacing of OMMT increased from 1.68 to 3.81 nm after being intercalated by POSS. Poly(L-lactide) (PLLA) based nanocomposites with montmorillonites were produced by melt compounding. The PLLA nanocomposites with POSS modified OMMT were comprised of a random dispersion of intercalated/exfoliated aggregates of layered silicates throughout the PLLA matrix. The incorporation of POSS modified OMMT resulted in a significant increase in both crystallization temperature and decomposition temperature for 5% weight loss in comparison with the virgin PLLA. Gas Permeation Analysis showed that the increase of the montmorillonite concentration in the polymer matrix led to an expected decrease in permeation values. Gas barrier properties of the nanocomposites were compared with those predicted by phenomenological models such as the Nielsen model and Cussler model. (C) 2013 Society of Plastics Engineers