The aim of this study was to investigate the effects of montmorillonite (MMT) type nanoclay content and maleic anhydride (MA) grafted polylactide (PLA) copolymer compatibilization on the mechanical properties of PLA nanocomposites; compounded by twin-screw extrusion melt-mixing followed with injection molding for shaping of bulk specimens. Successively intercalated and exfoliated structures of the nanocomposites were observed by XRD and TEM analysis, including the chemical interactions by FTIR. Mechanical tests indicated that use of only 1 wt% MMT resulted in significant improvements in many mechanical properties of PLA. Increases were only 14% in flexural modulus and 7% in flexural strength, but more than 6 times in ductility and as much as 85% in fracture toughness values. After MA compatibilization these improvements were much enormous, being more than 22 times in ductility and 119% in fracture toughness.