The Photochemical and photo catalytic reactions have been studied under ultra violet light. The effect of pH, effect of concentration, effect of amount of semiconductor and effect of semiconductor, effect of intensity of light have been observed. The results obtained during the course of present investigations clearly indicate that the photolysis of Cypermethrin is fastest in the ultraviolet light in the presence of semiconductors. The reaction is much slow without catalysts. A tentative mechanism has been proposed for photochemical reaction.