The pesticides are widely used in the agricultural sector to increase crop production. However, pesticides are considered as harmful chemicals to human health, animal, fish and may cause environmental damage including water pollution. This paper reports a study on the development of a fluorescent sensor system to detect pesticides in water using ZnCdSe quantum dots (QDs) thin film. An optical sensing system was set up, comprises of four components: light source to excite pesticide, sensor chamber that contains the sample of pesticide, fiber optic to drive light from source to sensor chamber, and analyzer system. The dual arms fiber optic probe is for excitation and emission light paths. A glass surface probe is coated with ZnCdSe QDs thin film and attached at the end of the probe arm. ZnCdSe QDs was synthesized at 5 min of growth time using the wet chemical process at 350°C. The detection of pesticide was done by comparing photoluminescence (PL) spectra of the ZnCdSe thin film in deionized water and then in the pesticide solution. The concentration of pesticide solution was prepared in water at 2.5 μg/L to 2500 μg/L. The sensing experiment showed that the PL peak spectrum of ZnCdSe thin film was dropped after dipped in the pesticide solutions compared with PL peak of ZnCdSe dipped in the water. The percentage drop of PL peak was increasing with the increasing of pesticide concentrations and linearly depends on pesticide concentrations.