Abstract: Using the self-made poly-clonal antibody, the affect of the ion-strength and the methanol concentration to the competitive reaction were investigated, and the reaction system was optimized. An indirect competitive enzyme-linked immunosorbent assay (ELISA) for the determination of parathion-methyl in cucumber was developed. The results showed that the concentration of parathion-methyl that inhibits 50% of antibody-antigen binding（I50）was 0.13 mg/L, the detection limit I20 was 6.7 μg/L. Most analogues showed negligible cross-reactivity except for fenthion and fenitrothion. Cucumber samples were detected with ELISA and GC after simple pre-procession. The fortified recovery of ELISA were 83.10%~93.46%，CV were 6.61%~10.36%，the determination limit was 0.003mg/kg. The results of the ELISA were verified by GC method.