A model polyethylene-poly(L-lactide) diblock copolymer (PE-b-PLLA) was synthesized using hydroxyl-terminated PE (PE-OH) as a macroinitiator for the ring-opening polymerization Of L-lactide. Binary blends, which contained poly(L-lactide) (PLLA) and very low-density polyethylene (LDPE), and ternary blends, which contained PLLA, LDPE, and PE-b-PLLA, were prepared by solution blending followed by precipitation and compression molding. Particle size analysis and scanning electron microscopy results showed that the particle size and distribution of the LDPE dispersed in the PLLA matrix was sharply decreased upon the addition of PE-b-PLLA. The tensile and Izod impact testing results on the ternary blends showed significantly improved toughness as compared to the PLLA homopolymer or the corresponding PLLA/LDPE binary blends.