A series of magnesium benzyl alkoxides, [LMg(mu-OBn)](2) (1-8), supported by NNO-tridentate Schiff-base ligands are prepared by the reactions of Mg(OBn)(2) with one molar equivalent of related Schiff-base ligands ((LH)-H-1-(LH)-H-8) in toluene. The molecular structures of compounds 2, 3, 7, and 8 are characterized by X-ray structure determination. All of these compounds are crystallized as penta-coordinated dimers, and the central magnesium atoms are bridged by the oxygen atoms of benzyl alkoxides. The catalytic activities of complexes 1-8 toward the ring-opening polymerization of L-lactide are investigated. Experimental results indicate that the reactivity of [LMg(mu-OBn)](2) (1-8) is dramatically affected by the electronic effect of the substituents on the Schiff-base ligands, and 4 has the highest activity among them.