Fenamiphos is a widely used organophosphate insecticide/nematocide in vegetables and bananas to control pathogenic nematodes. This work reports the phototransformation of Fenamiphos under different intensity of UV light in distilled water. The rate of photolysis of Fenamiphos followed apparent first order kinetics (Ct = C0e-kt). The rate constants calculated at 25°C and pH 6.9 were 14.47 10-3min-1 (HPK125W), 4.11 10-3min-1 (PL-S 9W/ 10) and 3.1 days-1 (sunlight).Apparent rate constant was proportional to I0.5 (Intensity of light).Half-life (t1/2) values calculatedwere 48min (HPK125W), 168.6min (PL-S 9W/10) and 223 days (sunlight). From this study, two photoproducts were identified and characterized using High Performance Liquid Chromatography/mass spectrometry (HPLC/MS). The plausible mechanism of photolysis involved is oxidation of sulphonamide group.