Bio-diesel fuel (BDF) has attracted increasing attention in recent years as an alternative to fossil fuel. Since as much as 10% of vegetable oil and fat is converted to glycerin as a by-product during the BDF manufacturing process, an effective and acceptable conversion process of glycerin to useful substances is becoming increasingly important. Hydrothermal decomposition experiments of glycerin with an alkali showed that glycerin could be converted into lactic acid with a high yield of about 90 mol%. In this paper, a kinetic model for lactic acid formation from glycerin was developed based on the proposed pathway, whereby glycerin is first decomposed to pyruvaldehyde, which is then converted into lactic acid. Kinetic analysis showed the activation energy of the conversion of glycerin into lactic acid was 174 kJ/mol. A comparison of experimental and calculated lactic acid yields showed that the experimental and calculated values were in good agreement, indicating that the kinetic model developed in this study is valid.