Glucose, as a model compound of cellulosic biomass, was used as a test material. Ca(OH)2 and NaOH were selected as alkali. Results showed that both NaOH and Ca(OH)2, can promote the formation of lactic acid in a hydrothermal reaction of glucose. In the case of the addition of NaOH, lactic acid was obtained with a good yield of 27% based on a carbon base at 300°C for 60 s with a NaOH concentration of 2.5 M. In the case of the addition of Ca(OH)2, the highest yield of lactic acid is 20%, which occurred at 300°C for 60 s with a Ca(OH)2 concentration of 0.32 M. The formation mechanisms of lactic acid from glucose were also discussed according to intermediate products identified. Lactic acid may be generated via formaldehyde, glycolaldehyde besides via the aldose having three carbon atoms in hydrothermal reaction which all formed by the reverse aldol condensation of hexoses.