Abstract: A method was developed for GC-MS determination of methyl tert-butyl ether (MTBE) in water and soil with sample treatment by purging and trapping. Sample of water or soil was collected by the method of related standards and stored at 25 ℃. The parameters of sample-treatment by purging and trapping were taken as follows: ① time of purging: 11 min; ② time and temperature of desorption: 2 min at 190 ℃. After separation by GC, MS analysis was carried out under the condition of EI and SIM. Linear relationships between values of peak area and mass concentration of MTBE in water and soil samples were kept in definite ranges, and values of detection limits (3S/N) were 0.10 μg·L-1, 0.26 μg·kg-1, respectively. Recovery obtained by standard addition method were in the range of 87.6%-103% with RSDs (n=5) in the range of 2.3%-7.2%.