The aim of this work was to study the effect of crystallinity degree on the thermal, mechanical, and fracture properties of poly(lactic acid)/organomodified montmorillonite (PLA/OMMT) nanocomposites. Samples with two different compositions (0.5 and 2.5% weight OMMT in PLA) were prepared by melt mixing in a twin-screw extruder and injection molding. An annealing treatment was applied to increase the percentage of PLA crystallinity. The thermal behavior was analyzed by differential scanning calorimetry (DSC), and the mechanical properties were determined via tensile tests and the fracture behavior using the linear elastic fracture mechanics theory, using SENB specimens at low and high testing rates. Gel permeation chromatography (GPC) from granulates and injected specimens were also carried out, finding some polymer degradation during extrusion and injection processes. The results show a toughening effect of the nanocomposites on amorphous specimens but embrittlement on the annealed ones. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 896-905, 2011