A method for determining lactic acid in tobacco by GC/MS was established with an Innowax capillary column (30m×0.25mm i.d.×0.25µm d.f.) and selective ion mode, using TMHS as a methyl esterification reagent and n-pentanol as an internal standard. The linear range of this method was from 9.144×10-3mg/mL to 121.92×10-3mg/mL lactic acid (r=0.9986). The average recovery of lactic acid was 98.09% and the RSD of this method was 1.83%. The contents of lactic acid in some tobacco leaves and tobacco products were determined with this method.