Hydrothermal treatment of glycerin was carried out at 300°C by using eight base catalysts, including hydroxides of alkali metals, alkaline-earth metals and aluminum. All selected catalysts promoted the conversion of glycerin into lactic acid, except for Al(OH)3. The alkali-metal hydroxides were found to be more effective than alkaline-earth-metal hydroxides on hydrothermal conversion of glycerin. In the case of NaOH or KOH, a lactic acid yield of approximate 90% can be approached. Moreover, KOH was superior to NaOH on promoting the hydrothermal conversion of glycerin. Ba(OH)2 and Sr(OH)2 was found to convert glycerin further into formic and acetic acids. A decomposition mechanism of lactic acid with Ba(OH)2 was proposed.