An efficient method for determining F8426 of Affinity 40 DF in soil and wheat is described. F8426 was extracted from the soil and the wheat sample with acetone-water (80:20, volume ratio), and the extract was partitioned with petroleum ether. Then the upper layer was concentrated and cleaned up on a micro-column of Florisil and activated carbon mixture. Finally F8426 was separated from other interfering components through wide-bore capillary gas chromatographic column (OV-1701, 12 m x 0.53 mm i.d.), and was detected by electron capture detector (ECD). The lowest detection limit for F8426 was 0.02 ng. The lowest detectable concentrations in wheat and soil was 2 micrograms/kg and 1 microgram/kg respectively. The average recovery and coefficient of variation of this method were 89.60%-97.53% and 4.42%-8.67% respectively. This method is simple, sensitive and suitable for residue analysis of F8426.