The effects of nanoparticles shape and surface modification on crystallinity and on gas and vapor properties of nanocomposites polylactide (PLA) films were evaluated. Films were prepared via solvent casting from PLA nanocomposites obtained by in situ polymerization of L-lactide using two different nanoparticles: nanosilica (NS) and an organically-modified montmorillonite (MMT), Cloisite 15A. To improve the compatibility between the polymer and the nanoparticles, NS and MMT were also modified with different amounts of two silanes, 3-aminopropyltriethoxysilane or 3-glycidoxypropyltrimethoxysilane. Thermal analyses indicate that nanoparticles (both with and without silanes on the surface) enhance crystallization processes and the effect of NS is much higher than the effect of MMT. The presence of NS nanoparticles, especially when modified with silanes, contribute in greatly enhancing crystallinity and in lowering permeability to O2 and CO2; values of O2 and CO2 permeability were reduced up to 80% and 50% respectively, compared to pure PLA; water vapor (WV) permeability is significantly affected by the presence of the nanoparticles but not by their shape and by modification with silanes. The in situ-synthesis permits to use lower amounts of nanoparticle in comparison to melt extrusion or blending processing techniques, in order to achieve the same or better performances, thanks to an improved dispersion of the fillers, that is obtained also due to the surface modification.