Abstract: Residual phoxim in welsh onion was extracted with acetonitrile at about 2 ℃,and the extract was evaporated to nearly dryness after salting out with sodium chloride.The residue was taken up with 1 mL of acetone and was ready for GC determination.Considering the thermoinstability of phoxim,the conditions of gas-chromatography were optimized to reduce its thermo-decomposition and to raise response value.Linear relationship between the peak area and concentration of phoxim was kept in the range of 0.10-10.0 mg·L-1 (r=0.999 9),with its detection limit of 0.01 mg·kg-1.Recovery tests were made by addition of standard phoxim in the concentration range of 0.02 to 0.10 mg·kg-1,and the values of recovery obtained were in the range 88.1%-105.0%,and precision tests were also performed,giving values of RSD′s (n=6) in the range of 2.6%-9.2%.