A series of poly(L-lactic acid) (PLLA)-b-poly(methyl methacrylate) (PMMA) block copolymers with well-defined chemical structures were synthesized by ring-opening polymerization followed by atom-transfer radical polymerization. These copolymers were investigated as effective A-b-C-type compatibilizers for poly(lactic acid) (PLA) and renewable poly(epichlorohydrin-co-ethylene oxide) (ECO) elastomer blends. Compared to the neat binary PLA/ECO blend, the PLLA-b-PMMA copolymers significantly improved the compatibility of the PLA and ECO phases, leading to enhanced interfacial adhesion and mechanical performance. Interestingly, by tuning the chain structure and adding different amounts of the copolymer, two different phase structures were achieved in the blends: a typical sea-island morphology of the blends with a low content of PLLA-b-PMMA block copolymers (<15%) and a unique tricontinuous phase morphology with a percolated PLLA-b-PMMA copolymer zone of the blend with the addition of 20 wt % asymmetric A181M868 block copolymer. Both the morphological structures endowed the blends with excellent toughness, including a high elongation at break (>260%) and non-broken behavior with an optimum impact strength above 63 kJ/m(2). Accordingly, two different toughening mechanisms were proposed for the blends with different phase structures. The PLLA-b-PMMA block copolymers provide highly efficient and versatile A-b-C-type compatibilizers to fabricate high-performance sustainable PLA-based materials for wide application prospects.