A microemulsion composition comprising biphenyldimethyldicarboxylate (BDD) and silybin or a derivative thereof, or a Carduus marianus extract containing silybin and derivatives thereof, as active ingredients; a co-surfactant; a surfactant; and an oil provides a synergistic therapeutic effect for liver diseases due to the complementary work of the two kinds of active ingredients having different working mechanisms without any adverse side effect or an antagonism, and high in vivo bioavailabilitie of both of the active ingredients, when orally administered.