Lactide, a six-membered dimeric cyclic ester of lactic acid, is a key building block of polylatic acid, a representative bio-based biodegradable polymer. As an alternative to the conventional lactide production process of a twostep polymerization and depolymerization from lactic acid, we developed a novel continuous and one-step synthesis of optically pure lactide from lactic acid under atmospheric conditions with SnO2-SiO2 nanocomposites as heterogeneous catalyst. In this catalytic process, lactide was obtained in vapor phase together with water vapor and the unreacted lactic acid. After optimization of crystallization process using ethanol solvent, lactide crystals with 99 wt% purity and a lactide yield of 78 wt% were obtained. Based on these results, an integrated process for high-yield polymer-grade lactide production from aqueous lactic acid could be constructed by combination of the heterogeneous catalysis and crystallization with ethanol, which is more environmentally friendly as compared to the conventional two-step prepolymer process.