Abstract: Plant-esterase can catalyse the substrate 1-naphthyl acetate to be decomposed to a fluorescent product, 1-naphthol. The fluorescence intensity of 1-naphthol was measured using a spectrofluorometric method. The inclusion of β-cyclodextrin enhances the fluorescence intensity of 1-naphthol. Thus increases the sensitivity of the determination. using this method, the determination of methamidophos in vegetables has been performed with the linear relationship in the range of 2~50 mg/L,and the detection limit of 05 mg/L.