Abstract: Gas chromatography equipped with a nitrogen-phosphorus detector (GC-NPD) was used in the field residue decline study and final residues of malathion in citrus and soil.Samples were extracted with acetone and the extracts were cleaned up by liquid-liquid extraction, followed by determination using GC.The recovery was from 90.1% to 97.1% for the citrus' pericarp, 85.0% ~89.0% for pulp, 85.3% ~90.5 for the citrus and 85.8% ~90.0% for soil with coefficients of variation of 2.06% ~6.79%,4.00% ~7.57%,2.47% ~4.74 and 4.94% ~7.06% respectively.The limit of detection was 1.79×10-11 g.The limit of quantification was 0.013 mg/kg for soil, 0.009 mg/kg for citrus, pericarp and pulp.The half-life of malathion was 7.86~12.16 days in citrus and 1.0~2.0 days in soil.When sprayed for 3 successive times at application rate of 0.30 g/L,at interval time of 20 d after the last application, the final residues of malathion in citrus' pericarp were between 0.026 mg/kg to 0.117 mg/kg, and the final residues of malathion in citrus'pulp were lower than 0.009 mg/kg.