Amphiphilic double-brush copolymers (DBCs) with each graft site quantitatively carrying both a hydrophilic poly(ethylene oxide) (PEO) graft and a hydrophobic polylactide (PLA) graft were synthesized, characterized, and further utilized as surfactants for the stabilization of miniemulsions. Well-defined PEO-b-PLA-based diblock macromonomers (MMs) with exo-norbornene (NB)-functionalized diblock junction were prepared by the synthesis of a PEO-based NB-functionalized alcohol via polymeric reaction, followed by ring-opening polymerization (ROP) of lactide (LA) initiated by the alcohol. Ring-opening metathesis polymerization (ROMP) of the MMs yielded DBCs. The well-controlled structures of the MMs and the DBCs were verified through rigorous instrumental characterizations. As compared with the MMs, the corresponding DBCs had lower crystallinities and melting temperatures (T(m)s) for both PEO and PLA phases and showed a negligible tendency for intermolecular self-assembly in solutions. With nanoscopic dimensions and novel amphiphilic architectures, these DBCs represent a new type of giant polymeric surfactant. Relative to the precursor MMs, the DBCs resulted in miniemulsions with remarkably enhanced stability.