Two magnesium and zinc complexes were synthesized from the reaction of magnesium chloride and zinc chloride with a tridentate NNO ligand in tetrahydrofuran. The magnesium and zinc complexes display good catalytic performance for the ring-opening polymerization of rac-lactide in the presence of benzyl alcohol and a polymer with narrower molecular weight distribution is obtained. The linear relationship between monomer conversion and number average molecular weight implied that the complexes were capable of catalyzing the living/controlled ring-opening polymerization of rac-lactide. Kinetic studies indicated that the ring-opening polymerization of lactide catalyzed by the catalyst contains two linear stages. In these two stages, the polymerization follows first-order kinetics for the monomer concentration. The first-order dependency of the catalyst concentration was observed in the first stage. Fractional dependency upon catalyst concentration was found in the second stage. The analysis of the end group structure of the polymer indicated that the benzoxy group and hydroxyl group are the end groups of the polymer. The study on the reactions between alcohols and the complexes implied that the complexes were stable in the presence of benzyl alcohol. The results showed that benzyl alcohol was the initiator of the polymerization and the ring-opening polymerization of rac-lactide conformed to the monomer activated mechanism.