A series of polylactides (PLA) with high molecular weights, various optical purities and thermal properties have been prepared using low-toxic zinc as catalysts. The effects of splitting temperature on optical purity of lactide were discussed. Low-toxic zinc lactate was used as catalyst for bulk ring-opening polymerization of lactide. The effects of optical purity of monomers on the stereo structure of PLA as well as the crystallization on the molecular weight and thermal property were investigated. The resulting monomers and polymers were characterized by polarimetry, 1H NMR, DSC and GPC. It was shown that high optical purity lactide and crystallization in polymerization led to higher molecular weight PLA using low-toxic zinc lactate as catalyst.