The photosensitized reaction of imidazole has been studied in the alkaline medium in the visible light using methylene blue as a photosensitizer. The rate of the reaction has been calculated. The effect of the parameters like pH, concentration of the sensitizer, concentration of the substrate and intensity of the light on the rate of the photosensitized reaction has been observed. The reaction has been studied in the anaerobic condition to observe the effect of the oxygen. Methanol shows free radical scavenging effect. The quantum efficiency of the photosensitized reaction has also been calculated. The scheme of the reaction mechanism has been suggested. The singlet oxygen (1O2) participates in the photoreaction and oxidized photo- product is formed. The reaction has been studied using UV-visible spectroscopy and product was analyzed Ion-Chromatography.