Abstract: Dichloroacetic acid and trichloroacetic acid in different water samples, such as surface water, sea water, sewage treatment plant influent and chemical waste water, were quantitated by GC. Sodium chloride was added to water samples then the samples were acidified by sulfuric acid, followed by extraction using methyl tert-butyl ether the mixture of sulfuric acid and methanol (20+80) was used as derivatization reagent at 50 ℃, and derivatization with methyl alcohol before determined using GC-ECD. Good separation of dichloroacetic acid and trichloroacetic acid was obtained with split ratio of 5∶1 under optimal condition. Detection limits of dichloroacetic acid and trichloroacetic acid were 0.21 μg·L-1 and 0.14 μg·L-1 respectively. Values of recovery found standard addition method were in the range of 97.4%-105%, with RSD (n=6) less than 6%.