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PLA bio-composites reinforced by oligo(d -lactic acid) grafted chitosan has been developed for simultaneously improved ductility, strength and modulus. Brittleness problem greatly limits the applications of PLA, a polymer derived from corn. Various methods have been developed to solve the brittleness problem. Unfortunately, these methods have their limitations, such as sacrifice of strength and modulus of PLA, use of toxic chemicals and high costs. Bio-based elastomers such as chitosan also have poor compatibility with PLA, leading to poor mechanical properties. The hypothesis for this research is that CS-g-oligo(D-LA) particles with good ductility could form strong interfacial interactions with PLLA matrix. Reinforcing effect of CS-g-oligo(D-LA) particles on PLLA matrix was systematically studied. Compatibility and intermolecular interactions between CS-g-oligo(D-LA) particles and PLLA matrix were studied by SEM, DSC and 13C NMR analyses. The reinforcing mechanism was summarized. Due to effective transfer of stress from PLLA matrix to the strong but ductile skeletons of CS-g-oligo(D-LA), ductility, strength and modulus of PLLA bio-composites were substantially improved. This novel reinforcing strategy via formation of strong interactions between enantiomeric lactyl units would enrich the fabrication and exploration of high-performance PLA-based bio-composites. Unlabelled Image • PLLA bio-composites reinforced by oligo(D-LA) grafted CS are developed. • Improvement in composite ductility coincides with increase in strength and modulus. • Such result is due to good properties of grafted CS and its compatibility to PLLA. • A small addition of oligo(D-LA)-g-CS is enough to achieve good reinforcing effect. [ABSTRACT FROM AUTHOR]